Dark Matter: One of the most curious and surprising facts about the Universe is that everything we ordinarily know makes up only about 5% of everything out there.
Me, you, our neighbors, stray cats, planets, stars… everything we can think of whose nature contains protons, neutrons and electrons contributes only to a tiny fraction of what the Cosmos houses. And that’s true even when we budget everything we’ve detected directly, from exotic subatomic particles to black holes.
The other 95% is divided into two components of which we know very little (to be optimistic and not say we know nothing): dark matter, which comprises about 27% of everything that exists, and dark energy, which encompasses the 68 % remaining.
To observe the Universe, in general, scientists have two different tools to investigate what is up there: the first is by looking directly at the electromagnetic radiation emitted and absorbed by matter, that is, by looking at the light that reaches our instruments; the second is through gravitation and its corresponding effects on matter (and one could also say the effects between matter, energy and the curvature of space).
The second tool is the one that tells us that dark matter exists, whatever it is. Its existence is inferred from its gravitational effects on matter and the light that we can observe, mainly in large-scale objects such as groups and clusters of galaxies. At these scales, for example, the effect of dark matter appears unmistakably in gravitational lensing, where the total amount of mass in these systems distorts the light from objects located in the background.