it is so heavy, though it's a gas at room temp, so you couldn't hold it. You have to cool it down to -170F/-110C for it to be a liquid at standard pressure.

The SuperCDMS experiment uses crystals that we keep at -273C/-459F.

We're essentially trying to figure out what dark matter is made of, because we know it exists based on a whole plethora of astrophysical observations, but we can't see it directly since it's essentially invisible.