In early 1990s, the scientists were pretty sure that the Universe is expanding. It might keep expanding and never stops, or it might have enough energy density to stop its expansion and re-collapse. But they were sure that gravity  will slow the expansion as time went on.
Then in 1998, the observations of Hubble Space Telescope of a very distant supernova showed that the Universe expansion was actually accelerating and not as everybody thought, and no one knew how to explain it, but there was something causing it.
So theorists came up with some explanations, they even doubted with Einstein’s theory of gravity that might be something missing, maybe a new theory not yet discovered, so they called the cause of this phenomena “Dark Matter”.

There are lots of information that scientists don’t know about Dark Energy, but they know how much Dark Energy there is because they know how it affects the Universe’s expansion.

Albert Einstein

Albert Einstein

Albert Einstein was the first person to realize that empty space is not nothing. At that time, before Edwin Hubble’s discovery, no one including Einstein knew that the Universe is expanding, and he found that his equation of general relativity did not work for a static universe. Einstein decided to add a factor, that acts like “anti-gravity” and prevents the universe from collapsing. He called this idea, which was represented as an additional term in the mathematical equation representing his theory of gravity, the cosmological constant. Which means that the space can have it’s own energy, because energy is a property of space, and this energy will cause the Universe to expand faster.

Once Einstein knew the universe was expanding, he discarded the cosmological constant. He later called it the “biggest blunder of his life,” according to his fellow physicist George Gamow.
Today astronomers refer to one theory of dark energy as Einstein’s cosmological constant. The theory says that dark energy has been steady and constant throughout time and will remain that way.

A second theory, called quintessence, says that dark energy is a new force and will eventually fade away just as it arose. If the cosmological constant is correct, Einstein will once again have been proven right about something even he thought was a mistake.

In conclusion, there are lots to explore about Dark Matter, and scientists are eager to find out more because it will reveal and solve so many puzzles and questions. Let us see what Professor Michio Kaku has to say about this issue: