The mystery of dark matter
There's more of the universe that we don't understand than we do understand. Ordinary matter—the stuff that scientists have spent decades studying—makes up around five percent of the universe. The remainder is thought to be comprised of dark energy (around 70 percent) and dark matter (around 25 percent). What is all this dark stuff and how do we know it's there if we can't even see it directly?
We know that dark matter exists because it acts on the cosmos in a number of ways. In the 1930s, an astrophysicist named Fritz Zwicky realized that, in order to act the way they do, galaxy clusters must contain a lot more mass than was actually visible. If the galaxies also contained unseen "dark" matter, everything made a lot more sense. Then, in the 1970s, astronomer Vera Rubin discovered that stars at the edge of a galaxy move just as quickly as stars near the center. This observation makes sense if the visible stars were surrounded by a halo of something invisible: dark matter. Since then, a number of other astronomical observations have confirmed the effects of dark matter.
Several dozen experiments are now on the hunt for stronger evidence that dark matter exists. Many of these experiments look for Weakly Interacting Massive Particles, or WIMPs. Others search for a particle called the axion, a theoretical neutral particle that interacts with other particles extraordinarily weakly, or theorized dark-matter versions of the photon.
Experiments generally hunt for dark-matter particles in two ways: either through a direct search in which dark-matter particles bump into target material and scatter off atomic nuclei, resulting in a measurable nuclear recoil (these experiments are usually located underground, where there's little background noise), or through an indirect search for particles that should appear if a dark matter particle annihilates (these experiments are generally conducted with ground-based or space satellite telescopes). It is also thought that if dark matter particles can annihilate into regular (or Standard Model) particles, then the reverse could be true, and that dark matter particles could be created during high-energy collisions like those at the Large Hadron Collider.