The central regions of galaxies are extremely crowded places, containing up to a few hundreds of millions of stars. They are generally extremely dense environments, where a variety of phenomena occur frequently.
The centre of the Milky Way is a very crowded region, hosting a dense and compact cluster of stars—the so-called nuclear star cluster—and a supermassive black hole (SMBH) weighing more than 4 million solar masses. A star cluster is an ensemble of stars kept together by their own force of gravity. These large systems are found in the outskirts of every type of galaxy, being comprised of up to several million stars.
Our current understanding of the Universe suggests that it is composed of an invisible component called “dark matter”. This mysterious type of matter represents more than 25% of the entire matter and energy of which the Universe is made. The matter that we are used to “seeing” in our everyday life and that represents the building blocks for both our bodies and stars that shine in the sky, represents only 5% of the Universe.