Dark MatterFor centuries, physicists and philosophers have wondered what makes up our universe. Aristotle thought that all matter came in one of four forms: Earth, Air, Fire, and Water. Since then we have come a long way, with the discovery of atoms and the subatomic particles of which they are made. We can also guess what constitutes protons and neutrons. We have since discovered and predicted the existence of particles other than the atom, such as the photon, neutrino, axion, and many others. Despite all our advances in particle physics and astrophysics, we still don't know what form of matter makes up 95% of the universe. Physicists have called this mysterious substance dark matter, because it cannot be detected by observation (it does not emit visible or other frequency light waves). However, we know that dark matter must exist, following Newton's law of universal gravity. There are two ways to prove the existence of dark matter. We know that the universe must have a certain mass for its gravitational attractive forces to slow down the expansion of the universe that began with the big bang. We can precisely calculate how fast the universe is expanding now and how fast it has expanded in the past. From this we obtain the theoretical mass of the universe. This figure is well below the visible mass of the universe, which consists of stars, planets and hot gas. In this way, scientists have managed to prove that we can only see about 5% of our universe. We can also demonstrate that dark matter exists in galaxies by examining how they rotate. When an object rotates in a circular orbit, the object has a tendency to fly away following a path tangential to the orbit. If it remains within the orbit, it has a radial acceleration equal to the square of its velocity over the radius of the orbit. The only force that keeps the body in orbit is the force of gravity, which depends on the mass of the system. Knowing this, physicists can calculate the mass of a galaxy by observing how fast the stars orbiting its center move. Physicists can also calculate where the highest percentage of dark matter should be in the galaxy. In most cases, it is located in a ring just outside the galaxy. In the case of the galaxy shown in the photo, dark matter must be present in the dark space between the core of the older, yellow stars and the outer ring of the young, blue stars.