Question

Describe what astronomers are talking about when they speak of dark matter and dark energy, and clearly distinguish between them. what fraction is dark matter and what fraction is dark energy? What observational evidence have astronomers amassed to propose the existence of these phenomena?

Answer 1

In the 1930 a researcher by the name Fritz Zwicky was investigating the movement of a particular galaxy in space around 300 million lightyears away from earth. He observed that the rotation of the galaxy was so fast that nothing should have been able to hold itself together. By his calculations there needed to be roughly 400 times the matter that was visibly present to at least hold the galaxy together. These galaxies were rotating faster than their escape velocity which means that the planets should have left the galaxy and be thrown out of orbit. But that didn't happen. He concluded that the galaxy must be having more gravity than what is calculated by visible celestial objects in the galaxy. This matter that we can not see was named dark matter.

Another astronomer Vera Rubin, while studying the individual stars in individual galaxies discovered that stars at the edge of the galaxy move at a speeds greater than the stars at the center. This doesn't make sense. Take our solar system for example, mercury moves at a higher speed than jupiter or saturn. At those speeds the stars technically should have been thrown out of orbit. This was another clue about dark matter.

Dark Matter : it is a hypothetical form of matter. It's presence was implied by astrophysical observations like gravitational effects which can not be present unless more matter is present. It doesn't emit, reflect or interact with any kind of electromagnetic radiation which makes it hard to observe.

While studying the death of a star (type 1-A supernovae in particular) scientists discovered that the amount of light and radiation emitted was actually lower for starts which were at increasing distances. This meant that galaxies are moving apart from each other at high speeds. Dark energy could as well just be a property of space. This idea states that empty space isn't actually empty space. It's energy. Dark energy is said to be everywhere.

Lastly about 25% is dark matter and 70% is dark energy leaving visible matter to only 5% of the universe.