Dark matter

First and foremost, we can define dark matter as invisible matter that does not interact with electromagnetic waves, i.e. neither emits nor absorbs light, and which, on the other hand, interacts gravitationally with its environment: this is how it came to be discovered.  First put forward in 1933, but never given the slightest attention, the theory of the existence of dark matter was born of strange observations that contradicted theoretical predictions based on the laws that govern our universe. In 1933, Swiss astronomer Fritz Zwicky observed a small group of 7 galaxies in the Berenice cluster. His observations revealed that the rotation speed of these galaxies was far too high for the predictions based on Newton's laws.

                                                                                                                                                                                                                                                                                             Fritz Zwicky

Nevertheless, in 1933, the precision of the measurements could not rule out potential errors. 40 years later, in the 1970s, Vera Rubin and Kent Ford came to similar conclusions, having studied the rotation speed of galaxies using a spectrograph (a device for breaking down the light emitted by stars) and relying on the Doppler effect, a phenomenon that exploits a difference in wavelength between that emitted and that observed in stars.

Vera Rubin                                                                                           

However, the evolution of a galaxy's rotation speed can be related to its mass distribution: the greater the mass in the galaxy, the faster its rotation speed. When we compare the theoretical and experimental rotation curves of the same galaxy, we notice the anomaly: they are different. At great distances from the galaxy's center, the theoretical speed decreases, while the experimental speed remains constant. This is how the study of galaxy rotation curves reveals a "missing mass" at a certain distance from the galaxy center. 

To explain this phenomenon, the theory of the existence of dark matter was born: dark matter would then form a halo of matter around the galaxy, offsetting the visible matter on which the galaxy's theoretical rotation curve is based. 

This is how Vera Rubin proposed the existence of dark matter, this time backed up by tangible evidence.

Today, this theory remains the most widely accepted, despite the fact that dark matter has never been directly observed. In fact, it can be detected through gravitational phenomena such as galaxy rotation curves and gravitational lens phenomena.

To give you some proportions, dark matter is estimated to make up around 25% of the universe, while ordinary, visible matter accounts for just 5%.

The rest of the universe is thought to be made up of dark energy, a kind of force opposed to gravitation, which is responsible for the expansion of the universe.