The dark matter !

So we already know that to keep a galaxy working the way it is some kind of invisible matter must exist to uniform the gravity. In fact, the calculations show that the amount of this invisible material must be about 85% of the existing tangible material to maintain the equilibrium.

We call it the dark matter and until today it's still considered one of the most mysterious riddles in astronomy this kind of matter is completely invisible and doesn't interact with light and eventually everything made of normal matter: protons, electrons, neutrons, molecules ... was eliminated.

One of the possibilities was thinking about Axions, these particles have mass so if they're gathered together into a galactic-sized cluster they'll have enough gravity to affect galaxies, they even don't seem to interact with light so even a huge cloud of them would seem as dark as space itself and unnoticeable to the naked eye.

Another interesting property about Axions is manifested in their strange interaction with normal matter;

If you take a tangible object and throw it onto another one they would collide and change their initial direction, but for Axions, the particles would pass through the materials like a ghost, it could be possibly passing by us right now this is what makes them really hard to be noticed so you must be wondering how do we know of their presence ?