Summary Most missing matter is gas dispersed in intergalactic space Phenomena called fast radio bursts help solve the mystery WASHINGTON, June 16 (Reuters) - The universe has two kinds of matter. There is invisible dark matter, known only because of its gravitational effects on a grand scale. And there is ordinary matter such as gas, dust, stars, planets and earthly things like cookie dough and canoes. Scientists estimate that ordinary matter makes up only about 15% of all matter, but have long struggled to document where all of it is located, with about half unaccounted for. With the help of powerful bursts of radio waves emanating from 69 locations in the cosmos, researchers now have found the "missing" matter. [full article]
If true then so much for PBH, WIMPs, etc. So dark matter becomes "you gotta squint really hard" matter. Clown college.
Large areas of matter have more gravity than it should. Large areas of space have more expansion than small local areas. Something to do with the fourth dimension.
Its helpful to know what is meant by 'hot' in interstellar space. Its basically the particles have high energy but low density, same as the 'wall of fire' around the solar system. https://www.iflscience.com/why-some-people-call-the-solar-systems-edge-the-wall-of-fire-72918?u