We’ve long known that we could not account for all of the matter in the galaxy—with its evidence visible through gravitational force but its presence simply not observable. Now, two different groups have made a substantial discovery: half of it can be attributed to
particles called baryons (rather than dark matter), which links galaxies together “through filaments of hot, diffuse gas,” according to New Scientist. The gas is not hot enough for x-ray telescopes to observe, however. So, until a new system was put to use through the Planck satellite, the baryon particles went unobserved. This doesn’t account for everything we can’t see, but it’s certainly a major development.