Happy Dark Matter Day! Meet the usual — and unusual — suspects in this cosmic mystery

Dark Matter Day is a time during which scientists celebrate the hunt for the universe's most mysterious stuff.

Baryonic matter accounts for just 15% of all stuff in the universe.

Dark matter is different because it doesn't interact with everyday matter and photons, or it does so in such a weak and rare way that we can't detect it.

Scientists are hunting for extensions to the Standard Model , which could contain potential dark matter candidates.

Axions are hypothetical particles that are lighter than WIMPs, but they still have qualities that make them good candidates for dark matter.

If they exist, axions wouldn't interact with photons and thus be "invisible" like dark matter, so they must exist in vast numbers.

MACHOs are celestial bodies that emit little to no radiation and drift through interstellar space alone, meaning they are too faint to be seen.

Primordial black holes could be a good fit for dark matter because they would be bounded by one -way light-trapping surfaces.

That would mean that these black holes, like dark matter, don't emit or reflect light.

But Stephen Hawking proposed that black holes "leak" a form of thermal radiation that was later named " Hawking Radiation " This should eventually lead to a black hole evaporating until it disappears in an explosion.