The first question people ask about dark matter is, “Why is it called dark?” In fact, dark matter isn’t dark like a black hole, which absorbs (or rather, sucks up) any light shined on it; instead, it’s dark simply because we cannot see it. Light of any and all wavelengths passes right through dark matter as if it wasn’t even there. Dark matter is functionally invisible. Except…
We know something is there. When we examine the structure and movement of galaxies and clusters of galaxies, they do not correspond with models build from luminous matter (meaning any matter that we can see in any wavelength of light). This lack of correspondence comes down to the fact that there is not enough matter in clusters to hold them together, or enough matter in the galaxies to cause them to spin the way they do (a third piece of evidence is gravitational lensing, but that deserves its own post because it is amazing all on its own). Since matter has gravity and gravity causes these effects, either there are properties of gravity that we don’t know about or there is matter that we can’t see. When it comes to the first observations that suggested dark matter, I’ll simply say that clusters and superclusters of galaxies are much more defined and “clumpy” than they ought to be if we only consider the matter we can see. Fritz Zwicky, a very interesting physicists and astronomer, first discovered this in 1933. The second reason we need dark matter is a bit more difficult to explain. To start with, here is a spiral galaxy:
As you can see, there are more stars (luminous matter) in the middle of the galaxy, and as you move outward there are fewer and fewer stars. Since there are fewer stars toward the edge of the galaxy, those stars should be moving slower than the stars in the middle of the galaxy, as there is less matter there and thus less gravity binding the outer stars to the inner ones. If they were orbiting the center of the galaxy too quickly, there wouldn’t be enough gravity to hold them in their orbits and they’d whizz off into space. Once again, though, there’s an Except… They are moving too quickly. The below chart is titled “Distribution of Dark Matter,” but it’s actually a simple rotation curve. On the vertical axis is the velocity at which the galaxy is rotating, and on the horizontal axis is how far from the center of the galaxy this velocity is measured.
The expected rotation curve is the one labeled “disk,” in which all of the matter in the galaxy is the matter we can see, which is mostly in the disk of the galaxy. As you move outward from the center, the rotation should slow. However, the actual observed rotation curve is the top one labeled “NGC 3196,” which is obviously different from what we would expect. As you move outward in the galaxy into the more sparsely-populated area, the speed of rotation actually remains the same. The galaxy is rotating much more quickly than it ought to be, so there must be an extra source of gravity that’s holding it together and allowing it to rotate like this without flying apart. This extra source of gravity is dark matter, shown in the curve labelled “halo” because it forms a sort of spherical halo around the galaxy:
This extra mass is what causes the unexpectedly consistent rotation of galaxies all the way out to their edges. While, from these observations, it’s clear that dark matter exists, physicists are not yet sure exactly what it is. The most popular theory at the moment is that dark matter is made of particles called WIMPs, which stands for Weakly Interacting Massive Particles: they are massive because their mass is the whole reason we know they exist, and they are weakly-interacting because they do not interact with light and just barely interact with regular matter. However, this weak interaction makes WIMPs by definition extraordinarily difficult to detect, as they we can only directly observe interactions with what we think of as “regular” matter, and WIMPs are very unlikely to interact with regular matter. Most major projects searching for dark matter involve some sort of underground tank that just sits and sorts the collisions that happen inside it, waiting for something that looks like dark matter.
There is no conclusive evidence yet to fully support the theory of WIMPs or any other theory of dark matter, so it may well be one of the biggest discoveries of this age in astrophysics and particle physics. Dark matter makes up about 27% of the Universe, compared to “regular” luminous matter’s 5%. Dark matter is out there, even though we can’t directly see it, and it is crucial to how our Universe functions.
If you want more information, Dr. Matthew Francis wrote a piece on this for the BBC which is rather more in-depth than this one and can be found here and Dr. Katie Mack wrote a fairly detailed and more experimentally-focused piece here. There’s also a more mathematically rigorous overview by Dr. David Bennett here. Dark matter is interesting and leads to some awesome effects, including gravitational lensing and also everything in the Universe not totally flying apart (at least not more quickly than it already is).