[Photo credit: NASA]
Throughout the history of the Universe, philosophers believed in the indiscernible presence of matter. Dark matter is the hypothetical invisible matter of the Universe, for it doesn't emit, reflect, or absorb electromagnetic radiation. Yet, its gravitational effect can be detected through stars and gases. It is believed that the Universe is made up of 27% of dark matter, 68% of dark energy and 5% of baryonic matter (normal matter). Unlike dark matter, baryonic matter emits, reflects and absorbs electromagnetic radiation. Baryonic matter is the ordinary matter that we can see in our lives. Dark energy is the hypothetical form of energy that is believed to behave opposite of gravity. Like dark matter, dark energy cannot be seen. Swiss astronomer, Fritz Zwicky, was the first person to prove the existence of dark matter. He studied the coma cluster of galaxies, which is made of thousands of galaxies and held together by the gravitational pull. He noted that they were moving faster than they were supposed to. This gave Fritz insight on invisible matter, now known as dark matter. One more clue that suggests the existence of dark matter is the virial theorem, which relates the total kinetic energy of a self-gravitating body due to the motion of its constituent particles.
[Photo credit: Cornell Astronomy]
The Galactic Rotation Curve, which is the difference in the orbital circular velocity of stars and gases at different distances from the centre, particularly provides substantial evidence for the existence of dark matter.
According to Kepler's Law, stars at the outskirts of the spiral galaxy should travel slower than those at the galactic centre. It has been proven that stars orbit at the same or greater speed regardless of where they lie in the galactic centre. The influence of dark matter can be detected by observing how the gravity of the massive galaxy clusters bends the light of the more distant galaxies found behind the cluster. This provides very compelling evidence for the existence of dark matter and is also known as gravitational lensing. There are many arguments against dark matter, and one of them is Modified Newtonian dynamics.
Modified Newtonian Dynamics claim that gravity does not depend on the mass of the object, but also depends on the gravitational pull of other massive objects in the Universe. If modified gravity is widely accepted in the future, scientists would no longer need to look for dark matter.
Early Evidence for Dark Matter: The Virial Theorem and Rotation Curves
A History of Dark Matter- Gianfranco Bertone & Dan Hooper
Shining a Light on Dark Matter- NASA