Lead

That's the stuff that makes up 85% of all matter in the universe, but we have no clue what it is.

A mile under Lead, South Dakota, you'll find a giant cauldron filled with seven tons of liquid xenon encased in water and surrounded by super sensitive photodetectors.

Known as the LZ detector, its job is to look for flashes of light that come from particles bumping into xenon atoms.

And over the course of seven months or so, the researchers say they saw one very weird flash, a bump that doesn't look like it could have come from any particle we know of.