The exact composition of 84 percent of all the matter in the universe is unknown. That is the portion, called dark matter, which neither emits radiation nor interacts with conventional matter that we already know of in any other way than through gravity. Cosmologists believe they can use the standard model of the universe, Lambda-CDM, to get to the bottom of dark matter. This model assumes that dark matter is “cold” (cold dark matter – CDM).
In physics, “cold” means that something is moving slowly. So-called “WIMPs” (weakly interacting massive particles) would have to be previously unknown particles, heavier than anything we know of so far, and would be detectable only by means of their gravity. The fact that dark matter is gravitationally active is basically the only thing we know about it for sure. In fact, that’s how its existence was determined in the first place.