We've seen this before. We know what it is.
~ Riker upon seeing the Crystalline Entity.
The Crystalline Entity was a powerful alien entity that inhabited the Star Trek universe, and originated in Star Trek: The Next Generation. Though not necessarily malevolent, it was a dangerous life form due to its alien physiology, which required it to constantly convert living matter into energy for it to absorb: it resembled a giant mass of crystals (hence its name) but was in reality a being made out of pure energy and it had the power to devour entire worlds or star ships and a number of inhabited colonies have fallen to this mysterious entity.

The Crystalline Entity can also be found in the MMORPG Star Trek Online, which takes place in an alternate-universe.