Galactic Mystery: Why bother with a galaxy that has no dark matter? Well, for starters, let’s understand what dark matter is. Dark matter is a type of matter that interacts gravitationally but does not interact electromagnetically. That is, it is a matter that does not interact with photons, an invisible matter that, however, affects the movement of non-invisible objects.
This interaction with visible objects (stars, galaxies, gas, among others) is what makes it possible to infer their presence. Speaking like this it sounds like a bit of science fiction, but the existence of dark matter is very well debated and widely accepted by the scientific community. That’s because we have about 5x more dark matter than the baryonic matter that makes up almost everything we see.
The presence of dark matter is critical to understanding galaxy formation! Simply put, at the beginning of the Universe, dark matter gathered together. Then baryonic matter (“normal” = gas and stars), fell into these regions, known as gravitational wells, collapsing and forming galaxies over millions – billions of years.
Now, in addition, the mass of these galaxies that formed is actually dominated by dark matter! It is now understood that galaxies are actually at the center of large spheres of this invisible material. That is, what we see is only around 10-20% of the real mass of these objects. And that was considered a rule, until now.