“A cosmic mystery of immense proportions, once seemingly on the verge of solution, has deepened and left astronomers and astrophysicists more baffled than ever. The crux… is that the vast majority of the mass of the universe seems to be missing.” -William J. Broad
Three classic observations — the Hubble expansion of the Universe, the leftover cosmic microwave background radiation, and the observed abundance of the light elements — lead us inevitably to modeling our Universe as having begun in a hot, dense state from which it expanded and cooled. There is no other explanation that agrees with the known laws of physics that can account for these three phenomena in detail.
But this doesn’t tell us what is in the Universe, at least, not on its own. Yes, it tells us there’s normal matter — protons, neutrons, and electrons — as well as radiation in the form of photons, and we can theoretically extrapolate that there need to be neutrinos as well, based on what we know about physics. But how much? And is there anything else? For that, we need to look at some specifics.
Specifically, when we look at the expansion rate, we look at not just what the expansion rate is now, but also what it was at every point in the Universe’s history.
We’ve made a whole host of observations, including spiral galaxies, elliptical galaxies, variable stars, surface brightness fluctuations, and type Ia supernovae, and they all indicate that the Universe’s expansion rate is about 68 km/s/Mpc, that it’s decreasing, and that it will asymptote to a minimum of around 55 km/s/Mpc in the far future. (This corresponds to just under 70% of the Universe’s energy being in the form of dark energy, or energy inherent to empty spacetime itself.) Because objects that aren’t already gravitationally bound continue to expand to greater distances over time (i.e., more “Mpcs”), they’ll continue to expand away at ever-increasing rates as the Universe ages.
We also look, in gory detail, at the abundances of the light elements. We have proof that the heavy elements we see today — Carbon, Nitrogen, Oxygen, and everything heavier — were all formed in stars, and that only the light elements hydrogen, helium, lithium and beryllium (and their isotopes) were formed in the hot, dense crucible of the early Universe. What the observed abundances tell us is that just under 5% of the total energy content of the Universe is in protons, neutrons and electrons, meaning that about 26% of the Universe is unaccounted for.
And thanks to the incredible observations of Planck (among others), we know that 26% needs to be dark matter.
But what could that dark matter be? From the cosmic microwave background (CMB) alone, we don’t learn very much about this matter other than it has to drop in density the same way normal matter does as the Universe expands, it cannot collide very much (if at all) with photons or normal matter, but it does clump and cluster together gravitationally.
Your first guess might be that this could be neutrinos, and for a long time, that was the leading theory.
The Universe is full of neutrinos (and anti-neutrinos) — some 1090 of them — left over from the hot, dense state of the early Universe. But we also have measurements of how many species of neutrinos there are (just 3), and measurements of the lower limits (a few hundredths of an electron-volt, from neutrino oscillations) and upper limits (about 2.2 eV-per-neutrino, from tritium decay) of neutrino masses.
If the Hubble expansion rate today were significantly lower, like around 50 km/s/Mpc, neutrinos might be a viable candidate from the CMB alone, but even the meager limits we presently have rule this out.
But we know more than this.
We can look into the early Universe at young quasars, or other ultra-luminous sources. In order to reach our eyes, that light needs to pass through all the clouds of gas that lie in between that light source and ourselves. And based on the absorption features we see in those gas clouds — how strong and how frequent the absorption lines are — we can conclude that this dark matter is not hot, which is to say that the speeds it’s moving at today are at most around 100 km/s; highly non-relativistic. (Or, in simpler terms, much slower than the speed of light.)
But there are other observations we can make.
We can look at the formation of large-scale structure in the Universe, at how galaxies clump and cluster together. These observations confirm that about 25% of the energy content of the Universe needs to be present in the form of dark matter. We see this whether we look at large-scale clustering from our deepest, largest sky-surveys, individual galaxies that are bound together in clusters, individually rotating galaxies, or galaxy/cluster pairs that are interacting or colliding.
But the large-scale structure observations tell us something even more.
They show us that there’s about a 5:1 dark matter-to-normal matter ratio, based on the magnitude of the “wiggles” you can see in the graph above! In fact, all our best observations, combined, point to the same picture of the Universe: mostly dark energy, most of the rest is dark matter, and about 5% is “normal” stuff like protons, neutrons, electrons, photons and neutrinos.
Now, you’ve likely heard some hype about dark matter being detected, and there’s a good reason for this: we know it’s there, but we don’t know exactly what it is. It could be a very, very low-mass particle (like an axion, in the micro-eV range) that was simply born cold (or slowly moving), or it could be a much higher-mass particle that was born very hot (or moving relativistically) and has slowed down as the Universe has cooled off. These could be anywhere from MeV-range (a few times heavier than an electron) to ~100 GeV-range (around the mass of the Higgs) all the way up to the Yotta-eV-range, or some 1024 eV, for a WIMPZilla!
None of the candidate particles that could be this dark matter are particularly compelling, which makes the puzzle all the more maddening.
Perhaps most frustrating is that, on small scales, the simplest model — that the dark matter is cold, collisionless, and completely non-interacting except gravitationally — gives predictions for too much dark matter at the cores of galaxies. In other words, the rotation curves we see should be slightly different if dark matter were truly cold and collisionless.
So maybe dark matter is warm, meaning that it’s moving faster than most models predict.
Or maybe it collides either with itself or with normal matter under certain conditions, which would mean that either we should find it annihilating with itself, emitting radiation, or ricocheting off of normal matter, all of which we can detect in principle.
Or maybe it’s completely cold and collisionless, but obeys a type of quantum exclusion rule that plays an important role at the cores of dark matter clusters.
Or — even more controversially — perhaps there are multiple different components of dark matter, some of which behave in different fashions from others.
Whatever the nature of dark matter, the evidence remains overwhelming in favor of it, but just what it is remains a mystery. Dark matter really matters, but don’t be fooled by those telling you we’re on the brink of finding it; that’s only true if it happens to be one of the few forms we’re actively looking for! Most of the candidates we’ve imagined — to be quite frank — we don’t even know how to detect in principle! So keep thinking, keep investigating and keep listening, but most of all, keep wondering at just what might be holding the largest structures in our Universe together!