What is Dark Matter ?

And they know some things dark matter can’t be.

We know that dark matter can’t be constituted by dim brown dwarfs or black holes. The main reason this doesn’t work is that we know the total mass dark matter brings into our galaxy, and it’s a lot, about 10 times as much as the visible matter.

If that amount of mass was made up from black holes, we should constantly see gravitational lensing events — but we don’t. It also doesn’t quite work with structure formation. And we know that neutrinos, even though weakly interacting, can’t make up dark matter either because they are too light and they wouldn’t clump strongly enough to seed galaxy filaments.

But we have no direct experimental evidence.

Despite decades of search, nobody has ever directly detected a dark matter particle and the only evidence we have is still indirectly inferred from gravitational pull.

Physicists have been looking for the rare interactions of proposed dark matter candidates in many Earth-based experiments starting already in the 1980s.

Physicists have been looking for the rare interactions of proposed dark matter candidates in many Earth-based experiments starting already in the 1980s.

Physicists have been looking for the rare interactions of proposed dark matter candidates in many Earth-based experiments starting already in the 1980s.

They are also on the lookout for astrophysical evidence of dark matter, such as signals from the mutual annihilation of dark matter particles. There have been some intriguing findings, such as the PAMELA positron excess, the DAMA annual modulation, or the Fermi gamma-ray excess, but physicists haven’t been able to link any of these convincingly to dark matter.

After everything the Universe has told us about itself, we’re convinced that some type of dark matter must exist: matter that’s different from any of the known particles in the Standard Model.

This dark matter outmasses all other particles and radiation in the Universe by a factor of five or so, but has yet to be directly detected. We know it exists, but we don’t know exactly what it’s made of. Until we do, this will remain a mystery in need of a more complete solution.