What is Dark Matter?
Here’s everything we know about Dark Matter:
(We don’t know anything about Dark Matter).
We don’t know, for example, if Dark Matter is matter at all. We assume it is only because “it” has gravity–but we don’t really know what gravity is either.
There are four forces that make our reality happen, and science understands three of them really well. All the “forces” in nature are caused by tiny particles of one type swapping particles of another type.
- There’s the electromagnetic force, which is carried by photons. Light, magnets, and electricity are all manifestations of this one.
- There’s the strong nuclear force, which is carried by gluons. Protons and Neutrons are made of quarks, and the strong force holds them together, both the quarks to make a proton, and the protons to make a nucleus.
- There’s the weak nuclear force, which is carried by W and Z bosons. The weak force creates radioactive decay and nuclear fusion.
- Then there’s gravity. We don’t know what particle carries gravity, and none of our particle accelerators have been kind to the theories that make a proposition. Everything we find in our experiments validates the Standard Model, which works by pretending gravity does not exist.
Even though Einstein did a great job showing us how gravity works at a cosmological scale, we still have no idea how gravity actually happens. It’s a mystery. What we know is that some particles have mass (from something called the Higgs field), and that anything with mass attracts other particles–even massless particles like photons.
When we count all the matter in a galaxy, all the stars, planets, comets, black holes, etc., there is not enough gravity to hold them together. The stars are all moving fast enough that galaxies should fly apart, but they don’t. We first noticed this in the 1930s, and we still don’t know why. We just call it Dark Matter.
Probably the most popular hypothesis among scientists is that Dark Matter is made of WIMPs (Weakly Interacting Massive Particles). Since Dark Matter is like 85% of all matter, we’re probably swimming in the stuff right now. But, we can’t tell—the only force we have that interacts with WIMPs is gravity, which is rather inconveniently the only force we don’t understand. One hypothesis that makes a lot of sense to me is that WIMPS came from ancient black holes that have evaporated via Hawking Radiation.
Shortest answer: Dark Matter is some unknown force that holds galaxies together when they should be flying apart.
Dark Energy, on the other hand, is some unknown force causing the Universe to expand, even though it should be contracting thanks to all that Dark Matter. :)
MinutePhysics is a great place to go for a really easy primer on concepts in physics. Here’s the one for Dark Matter:
Neil deGrasse Tyson has a great bit on Dark Matter too:
Lawrence Krauss has a book meant for “popular audiences,” but it’s still a challenging read. Good for those who really want to go dancing in the dark.