[ELI5] What exactly is luminous matter?
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Luminous matter is kind of the opposite of dark matter. When measuring the mass of a galaxy, the easiest thing to do is count all the very luminous stars. Since stars make up ~99.98% of the matter in a solar system, counting the stars will get you about that same percentage of the mass in a galaxy. And if you know 99.8% of the matter in the galaxy, you should also know the source of 99.8% of the *mass*.
That is *not* the case. Something is increasing the mass of galaxies which is *not* luminous. Originally, the idea was that it was literally physically dark stuff like brown dwarfs or large rocky planets. However, more research showed that there cannot be enough of that kind of dark stuff. There must be another explanation.
So, luminous matter = all the stars and black hole accretion disks and bright shiny *stuff*. As opposed to dark matter, which is something else that provides the majority of the mass in galaxies.
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