5 undeniable, truthful facts about dark matter

Every so often, advocates of a fringe theory — one that doesn’t fit the evidence as well as the mainstream theory — do what they can to breathe life back into it. Sometimes new evidence has come to light, legitimately challenging the mainstream theory and demanding that previously discarded alternatives be re-evaluated. Sometimes, a surprising set of observations supports a once-discredited theory, bringing it back into prominence. And at other times, it isn’t new data that elevates a contrarian viewpoint, but rather a false narrative is the culprit, as disingenuous arguments that have been rightfully dismissed by mainstream professionals suddenly take hold among either a new generation of inexperienced individuals, or outsiders who haven’t been exposed to the wide array of mainstream facts that supports the consensus position.

Unless you yourself have the necessary expertise to diagnose what’s being presented accurately and fully, it’s virtually impossible to tell these scenarios apart. Over the past few years, these disingenuous arguments have gotten more and more popular. One example is a well-known contrarian physicist who suggested, in text and in video, that the situation surrounding dark matter has changed, and that modified gravity now deserves equal consideration. Even more recently, another prominent physicist has stated a similarly dubious case for the non-existence of dark matter.

Unless you’re in the business of ignoring the majority of the cosmic evidence that we already have, however, you’ll find that those assertions are simply not the case. Here are five truths about dark matter that, once you know them, can help you see through the false equivalencies presented by those who would sow undue doubt about one of cosmology’s biggest puzzles.

Distant sources of light — from galaxies, quasars, and even the cosmic microwave background — must pass through clouds of normal matter. The absorption features we see enable us to measure many features about the intervening gas clouds, including the abundances of the light elements inside and the degree of ionization.

1.) The total amount of “normal matter” in the Universe is unambiguously known.

You might look out at the Universe — full of stars, galaxies, gas, dust, plasma, black holes, and more — and wonder if there isn’t more of the “known stuff” out there. After all, if there are additional gravitational effects over and above what we can account for, perhaps there’s just some unseen mass out there responsible for it. This idea, of “normal matter that’s just dark,” was one of the major ideas that stood in the way of dark matter becoming an accepted part of mainstream cosmology in the mid-to-late 20th century.

After all, there’s plenty of gas and plasma out there in the Universe, and you might imagine that if there’s enough of it, we wouldn’t need some fundamentally new type of matter at all. Perhaps, if neutrinos were massive enough, they could take care of that portion of the missing energy budget. Or perhaps if the Universe were born with too much matter, and some of it collapsed to form black holes early on, that could solve the cosmic mismatch we see.

But none of those things are possible, as the total amount of normal matter in the Universe is unambiguously known: a mere 4.9% of the critical density, with an uncertainty of just ±0.1% in that value.

From beginning with just protons and neutrons, the Universe builds up helium-4 rapidly, with small but calculable amounts of deuterium, helium-3, and lithium-7 left over as well. Until the latest results from the LUNA collaboration, step 2a, where deuterium and a proton fuse into helium-3, had the largest uncertainty. That uncertainty has now dropped to just 1.6%, allowing for incredibly strong conclusions.

The key observational restriction is the observed abundances of the light elements:

• hydrogen,
• deuterium,
• helium-3,
• helium-4,
• and lithium-7.

Over the first few minutes of the hot Big Bang, these light elements were forged in the early Universe’s nuclear fires: when conditions were hot and dense enough to permit nuclear fusion’s key reactions among the light nuclei.

The amount of each element we get is highly dependent on how much total normal matter there was back in those early moments, and specifically depends on one property: the baryon-to-photon ratio. It’s that one parameter that leads to the predicted abundances, and since we’ve measured the radiation density of the Big Bang’s leftover glow, measuring the abundances of the light elements can inform us very clearly about what the baryon density of the Universe is.

As of today, those abundances have been directly determined through spectroscopic measurements of gas clouds, and also indirectly: through detailed observations of the cosmic microwave background. Both types of measurements point toward the same picture: one where 4.9% ± 0.1% of the Universe’s energy is in the form of normal matter.

It tells us that black holes that form from baryons can’t be a significant fraction of the dark matter. We know how Big Bang Nucleosynthesis depends on neutrinos, and three types — the electron, muon, and tau — are the only ones allowed, so neutrinos can’t be the dark matter either. Nothing in the Standard Model, in fact, will do the job. But this key fact cannot be rightly disputed: given the amount of normal matter we’ve determined that we have, a new type of fundamental ingredient must exist to be consistent with our cosmological observations. We call this ingredient “dark matter,” and it must exist.

The largest-scale observations in the Universe, from the cosmic microwave background to the cosmic web to galaxy clusters to individual galaxies, all require dark matter and dark energy to explain what we observe. While the equations that govern the evolution are well known, as are the magnitudes of the initially overdense regions in our Universe, obtaining the necessary small-scale resolution to tease out the masses and properties of the smallest, earliest galaxies remains difficult.

2.) You cannot explain either the cosmic microwave background or the large-scale structure of the Universe without dark matter.

Imagine the Universe as it was back in the earliest stages: hot, dense, almost perfectly uniform, and expanding and cooling all the while. Some regions, born with slightly greater densities than others, will begin to preferentially attract matter to them, trying to gravitationally grow. Because gravity is always attractive, it’s the densest initial regions that will preferentially draw more and more of the surrounding matter into them.

As gravitation gets to work, the density starts to increase, just as you’d expect. This has another effect, however: it causes the radiation pressure inside to increase also. This growth can’t last forever early on, as radiation carries lots of energy and exerts a significant amount of pressure. Eventually, the gravitational growth causes the density to reach a peak, which causes photons (the particles of radiation) to flow out of that region, which causes the density to then go back down.

As time goes onward, larger regions can start to grow via collapse, while the smaller regions collapse, then rarify, then collapse again, etc., in an oscillatory fashion. This behavior will lead to a spectrum of temperature imperfections in the Big Bang’s leftover glow, and eventually will form the seeds of structure that grow into stars, galaxies, and the cosmic web. But you’ll get a different set of behavior, in both the cosmic microwave background and the Universe’s large scale structure, dependent on whether you have both dark matter and normal matter, or just normal matter alone.

The map (top) of the temperature fluctuations in the CMB from Planck, along with the temperature fluctuation power spectrum (middle) as measured. The bottom two panels show the simulated temperature fluctuations on various angular scales that will appear in the CMB in a Universe with the measured amount of radiation, and then either 70% dark energy, 25% dark matter, and 5% normal matter (left), or a Universe with 100% normal matter and no dark matter (right). The differences in the number of peaks, as well as the peak heights and locations, are easily seen.

The reason these two scenarios — with and without dark matter — create two different spectra for temperature fluctuations is because the physics is different between the two cases. Sure, they have things in common.

• Dark matter and normal matter both gravitate.
• Increases in their densities both lead to increases in radiation pressure.
• And radiation flows out of an overdense region whether it’s made of normal matter, dark matter, or both.

But normal matter both collides with other normal matter and interacts with photons, while dark matter is invisible to it all: it doesn’t collide, stick together with, or exchange energy with normal matter or photons. As a result, a Universe with dark matter has twice the number of fluctuation peaks-and-valleys in both the cosmic microwave background’s spectrum and also the power spectrum of large-scale structure than a Universe with normal matter alone.

Definitively and unambiguously, dark matter is required. Specifically, that dark matter has to be cold, collisionless (at least with normal matter and photons, although it could potentially have collisions with itself), and invisible to electromagnetic radiation. In other words, it cannot be normal matter, and furthermore, cannot be any particle that’s a part of the Standard Model. If you want to turn up the dial on your skepticism-meter, keep an eye out for contrarian papers that attempt to explain either the cosmic microwave background or the matter power spectrum without dark matter; chances are that they add something in — like a massive neutrino, a sterile neutrino, or an extra field with a specifically tuned coupling — that functions indistinguishably from how dark matter would behave.

While the web of dark matter (purple, left) might seem to determine cosmic structure formation on its own, the feedback from normal matter (red, at right) can severely impact the formation of structure on galactic and smaller scales. Both dark matter and normal matter, in the right ratios, are required to explain the Universe as we observe it, with dark energy needed to explain how the expansion rate has evolved over time. Structure formation is hierarchical within the Universe, with small star clusters forming first, early protogalaxies and galaxies forming next, followed by galaxy groups and clusters, and lastly by the large-scale cosmic web.

3.) Dark matter behaves as a particle, and that’s fundamentally special compared to something that behaves as a field.

There’s another disingenuous narrative being peddled recently by those who wish to sow doubt about dark matter: that, because particles are just excitations of quantum fields, that adding a new quantum field (or modifying the gravitational field) can be equivalent to adding new (dark matter) particles. This is the worst kind of argument: one that has a technical kernel of truth to it, but that misleads about the core point of the matter.

The core point is this: fields are extremely general entities in physics, and they permeate all of space. They can be homogeneous (the same everywhere) or clumpy; they can be isotropic (the same in all directions) or they can have a preferred direction; they can couple to certain other sectors (i.e., have interactions) or can be decoupled from them (i.e., be non-interacting). Particles, by contrast, can be massless, in which case they must behave like radiation, or they can be massive, in which case they must behave like the massive particles we’re familiar with. If we’re dealing with a system of particles that are massive, then these particles will:

• clump,
• gravitate,
• exhibit the known, understood relationships between kinetic and potential energy,
• have meaningful particle properties like cross-sections, scattering amplitudes, and couplings,
• and behave according to (at least) the presently known laws of physics.

This snippet from a structure-formation simulation, with the expansion of the Universe scaled out, represents billions of years of gravitational growth in a dark matter-rich Universe. Over time, overdense clumps of matter grow richer and more massive, growing into galaxies, groups, and clusters of galaxies, while the less dense regions than average preferentially give up their matter to the denser surrounding areas. The “void” regions between the bound structures continue to expand, but the structures themselves do not.

It is for these reasons — for all of the properties of dark matter that we’ve been able to infer from astrophysical observations alone — that we conclude that dark matter is particle-like in nature. That doesn’t mean it can’t be:

• a pressureless fluid,
• a type of clumpy dust,
• or that its cross-section must be zero under every interaction except the gravitational one.

What it does mean, however, is that if you try to replace the scenario of dark matter particles with a field, there are constraints on how that field must behave. In particular, the field you’re replacing dark matter with must behave, from an astrophysical perspective, in a way that is not distinguishable from the behavior of a large set of massive particles.

Dark matter doesn’t have to be a particle, but to say, “It can be a field just as easily as it can be a particle,” glosses over the big truth: that dark matter behaves in exactly the fashion that we’d expect a new population of cold, massive, non-scattering particles to behave. You can have it be a field, but only the type of field that indistinguishably mimics the behavior of dark matter particles. Particularly on large cosmic scales, i.e., the scales of galaxy clusters (about ~10-20 million light-years) and larger, this particle-like behavior can only be substituted for with a field that behaves indistinguishably from how particle dark matter would.

Star formation in tiny dwarf galaxies can slowly “heat up” the dark matter, pushing it outwards. The left image shows the hydrogen gas density of a simulated dwarf galaxy, viewed from above. The right image shows the same for a real dwarf galaxy, IC 1613. In the simulation, repeated gas inflow and outflow causes the gravitational field strength at the centre of the dwarf to fluctuate. The dark matter responds to this by migrating out from the centre of the galaxy, an effect known as ‘dark matter heating.’

4.) Very real small-scale physics effects, like dynamical heating, star-formation and feedback, and nonlinear effects must be worked out and accounted for.

The problems with dark matter — or rather, the cases where cold, collisionless dark matter makes predictions that conflict with observations — almost exclusively occur on small cosmic scales: scales of large individual galaxies and smaller. It’s true: certain modifications to gravity can better match many of the observations on these scales. But there’s a dirty secret here: there’s messy physics on these small scales that everyone agrees has not been properly accounted for. Until we can properly account for them, we don’t know whether to call modified gravity or dark matter approaches successes or failures.

This is hard work! When matter collapses into the center of a massive object, it:

• sheds angular momentum,
• heats up,
• can trigger star formation,
• which leads to ionizing radiation,
• which pushes the normal matter from the center outward,
• which gravitationally “heats up” the dark matter in the center,

and all of this needs to be calculated. Furthermore, we’ve only been considering the simplest dark matter scenario: purely cold and collisionless, with no external interactions or self-interactions. Sure, we could modify gravity in addition to adding cold, collisionless dark matter, or we could ask, “What interaction properties could dark matter have that would lead to the small-scale structure we observe?” These approaches are equally valid, but both require the existence of dark matter — whether you call it dark matter or not — and must reckon with these known, real effects.

Importantly, individual galaxies don’t necessarily offer support for modified gravity over dark matter, and that if you try and infer galactic properties from the normal matter distribution alone, it only succeeds for certain sub-types of galaxies that exist, not for every type of galaxy that we find out there in the Universe.

Across a wide range of masses, galaxies all fell along a relationship called the baryonic Tully-Fisher relation, where the observed/inferred rotational speed was determined empirically by the normal matter alone, irrespective of dark matter. The existence of a population of galaxies that does not follow this rule, as shown with the orange stars, provides strong evidence for a fundamentally different population: a set of galaxies without dark matter, following the grey line.

5.) You must explain the full suite of cosmological evidence, or you’re cherry-picking, not doing legitimate science.

This is an enormous point that cannot be emphasized enough: we have all of this data about the Universe, and you must take all of it into account when you’re drawing your conclusions. This includes the following examples:

• you must look at all seven acoustic peaks in the cosmic microwave background, not just the first two,
• you must be honest about whether the alternative “thing” you’re adding (instead of dark matter) is equivalent to and indistinguishable from dark matter,
• you must not modify your law of gravity in a way that explains small-scale features at the cost of failing to explain the large-scale features that are successfully explained by dark matter,
• you must not pick statistically unlikely outcomes that have clearly occurred (but are not forbidden) as “evidence” that the leading theory is wrong (see the low quadrupole/octupole in the CMB for years of wasted effort on this front),
• and you must not oversimplify and mischaracterize the successes of the leading theoretical idea your contrarian approach wishes to supplant.

Remember, in order to overthrow and supersede an old scientific idea, the first hurdle you have to clear is reproducing all of the successes of the old theory. We may indeed need a new law of gravity to explain our Universe, but you can’t do it in such a way that dark matter isn’t also required.

The X-ray (pink) and overall matter (blue) maps of various colliding galaxy clusters show a clear separation between normal matter and gravitational effects, some of the strongest evidence for dark matter. The X-rays come in two varieties, soft (lower-energy) and hard (higher-energy), where galaxy collisions can create temperatures ranging from several hundreds of thousands of degrees up to ~100 million K. Meanwhile, the fact that the gravitational effects (in blue) are displaced from the location of the mass from the normal matter (pink) shows that dark matter must be present. Without dark matter, these observations (along with many others) cannot be sufficiently explained.

There are some very important points that you should never forget when it comes to the question of dark matter and modified gravity on both small and large scales. On the largest of cosmic scales, gravitational effects are the only ones that matter, and represent the “cleanest” astrophysical laboratory for testing cosmological physics. On smaller scales, stars, gas, radiation, feedback, and other effects arising from the physics of normal matter plays an enormously important role, and simulations are still improving. We have not yet reached the point where we can do small-scale physics unambiguously, but the large-scale physics has been there for a long time, and decisively points the way to dark matter and away from all modified gravity approaches that behave distinctly from dark matter approaches.

The easiest way to fool yourself is to do something that gives you the right answer without taking the full suite of what must be at play into account. Getting the right answer for the wrong reason — especially if you can check that the answer is right — is the most surefire way to convince yourself that you’re onto something big, even if the only thing you’ve captured is the effects of the important physics you’ve failed to consider. While we don’t know whether the law of gravity will ultimately need to be modified or not, we can be confident that, when it comes to the matter in our Universe, about 85% of it really is dark.

This article was first published in August of 2022. It was updated in October of 2025.