rationalskepticism.org

Sorry, Astronomers: Practically All Of The Universe's Matter Is Still Missing
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In addition to that, there's also evidence for matter — normal matter, made of protons, neutrons and electrons — existing in the space between galaxies and galaxy clusters: the warm-hot intergalactic medium. This ionized plasma has been very difficult to detect, but has long been thought to exist in large quantities, making up significantly more mass than all the stars in the Universe combined.

Recently, to the highest precision ever, this sought-after matter has been detected as pulses of light known as fast radio bursts travel through them on their way to Earth. This is the "missing matter" that's finally been discovered, as reported in numerous outlets over the past week or two. It's an extremely important discovery for astrophysics, but it doesn't come close to solving the problem of what or where the actual "missing mass" in the Universe actually is.
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