Dark matter holds galaxies together and gives the visible universe its structure. Even though it makes up about five-sixths of all the mass in the cosmos, to date nobody has been able to figure out what it’s made of. On the other hand, there have been some indications about what dark matter is not made of, but researchers still need to determine if dark matter is hot, cold, or possibly even fuzzy, with the temperature designation here referring to the speed at which the particles of dark matter are moving.
NASA’s Hubble telescope has now pushed the probabilities a little closer toward cold dark matter, which is also what the standard cosmological model (Lambda CDM) assumes. Using the Hubble, researchers were able to detect much smaller clumps of dark matter than was previously possible. If dark matter existed only in large clumps, which is what had to be assumed before based on observations, scientists had to come up with an explanation for the lack of smaller concentrations. This explanation was warm dark matter with quickly moving particles that would not stay together very long in small clumps. However, since these small clumps have now been detected in some sense, warm dark matter is no longer needed.