In the 1990s physicists were developing ideas about the current size and the rate of expansion of the universe. There were 2 realistic possibilities regarding expansion. One suggestion was that the universe possessed enough energy density (amount of energy stored in a given space) to stop expanding and recollapse (a ‘Big Crunch’), ending up as a single black hole whereby another big bang would recreate the universe. The other possibility was that the universe had the necessary energy density to never stop expanding. http://hubblesite.org/hubble_discoveries/dark_energy/de-fate_of_the_universe.php While there was much debate between these 2 ideas, physicists agreed that gravity was inevitably slowing down expansion. Any mass in the entire universe has a force of gravity acting upon it due to the fact that all gravitational fields technically expand forever. Scientists believed that this force of gravity was pulling all matter in the universe together and thereby was slowing expansion. However, in 1998, physicists began to observe distant supernovae through the Hubble Telescope and discovered that expansion was actually speeding up. That threw a spanner in the works.
If it’s not gravity which is pulling the universe apart then what is? Due to Einstein’s theory of relativity and his equation E=mc2, we know that matter and energy are interchangeable; merely different forms of the same thing. This is why all masses have a gravitational field force: they all consist of matter and therefore energy. So if some unknown energy is pulling apart the universe it must be some form of matter (or radiation). Yet we are suggesting that in space, even when there is no matter or radiation, there is some energy. The most popular theory for this solution is called the cosmological constant. This suggests that a constant energy density is filling space: when the universe expands and new existence is created, an energy force fills this new space. Physicists have coined this energy ‘dark energy’ and it is believed to make up 68.3% of the universe.
(Famously, Einstein described his belief that the Universe has a ‘cosmological constant’ of 0 as the greatest blunder of his career)
Dark matter is believed to make up 26.8% of the universe. Dark matter (as the name suggests) cannot be seen: it neither emits nor absorbs light, nor any other electromagnetic radiation. Its existence and its properties are inferred from its gravitational effects on visible matter (which makes up a mere 4.9% of the universe). Again, as the name suggests, dark matter is different to dark energy as it consists of matter and mass. In studying the gravitational field lines between massive astronomical objects, astrophysicists have deduced that some matter must be making up for the apparent discrepancies observed. The numbers simply do not add up: something else must be contributing to the mass of the universe, something we cannot see. According to cosmologists, dark matter is composed of a subatomic particle which has not yet been categorised. The search has begun.
(A supernova like the ones that helped physicists discover the universe’s expansion was accelerating)