Dark Matter :: physics science space

Dark Matter For centuries, physicists and philosophers alike 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 the atoms and the subatomic particles they are made of. We can even guess at what makes up protons and neutrons. We have since then 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 named this mysterious substance dark matter, for it can not 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 universal law of gravity. There are two ways to prove the existence of dark matter. We know that the universe must have a certain mass in order for its attractive gravitational forces to slow the expansion of the universe which started at the big bang. We can precisely calculate the rate at which the universe is expanding currently, and how fast it has expanded in the past. From this we get the theoretical mass of the universe. This figure falls far short of the visible mass of the universe, which consists of stars, planets, and hot gas. This is how scientists are able to prove that we can only see about 5% of our universe. We can also prove that dark matter exists in galaxies by examining how they spin. When an object rotates in a circular orbit, the object has a tendency to fly off in a path tangent to the orbit. If the stays within the orbit, it has a radial acceleration which is equal to its velocity squared over the radius of the orbit. The only force which is keeping the body in the orbit is the force of gravity, which is dependent on the mass of the system. Knowing this, physicists can calculate the mass of a galaxy by looking at how fast stars orbiting its center are moving. 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 nucleus of older yellow stars and the outer ring of young, blue stars.