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Dark Matter and Dark Energy:

Despite their similar sounding names, dark matter and dark energy are not related.

Dark matter is a hypothetical kind of matter that cannot be seen with telescopes. It doesn't emit or absorb light or any other kind of electromagnetic radiation--hence the name dark. Despite being essentially impossible to see, scientists think they have detected it indirectly using gravity. All matter feels the effects of the gravitational force and dark matter is no different. When a scientist observes a large astronomical object like a galaxy, he/she can deduce the object's mass based on the gravitational forces in the area. Then, when he/she looks at the object with an astronomical instrument any mass that can't be seen is considered to be dark matter by the process of elimination.

Dark matter hasn't been definitively proven to exist but the majority of scientists believe in it based on this indirect evidence.

Dark energy is a hypothetical kind of energy that is thought to permeate space. It basically works in a manner opposite to gravity, pushing matter apart rather than drawing it together. Dark energy was introduced to explain the observed acceleration of the expansion of the universe. Data has proven that the universe is expanding, but the acceleration of this expansion was a surprise to scientists.

To be honest, dark energy is kind of a fudge factor to explain why observations are not as expected. The famous physicist Albert Einstein originally included a dark energy term, called the cosmological constant, in his theory of General Relativity, but ended up throwing it out. Later, he said this was 'the biggest blunder of his life.'