You have probably heard many times about such a mysterious thing as dark energy. Many scientists talk about it, but you don't know what it is? Let's take a look at this issue.
How it all started
For a better understanding of the issue, you should start with a few other things. There are unique objects in our universe called standard candles. This is a type of supernova (Ia) (follow the link!) That occurs in binary systems of a dwarf star and a red giant. A neutron star captures the matter of a neighbor, and after reaching a critical mass, it flares up with a bright explosion. All these objects have one feature in common - they all shine with the same intensity. That is, thanks to them, we can fairly accurately measure distances to the most distant boundaries of the observable universe.
Observing these objects in 1929, Edwin Hubble discovered that the universe was expanding. He understood this based on the analysis of the incoming spectrum from these stars. If objects move away from us, then their radiation will shift to the red part of the spectrum, this is a consequence of the Doppler effect. The reason for this was the explosion that gave birth to the universe - the so-called Big Bang. Based on the then existing concept, it was expected that this expansion should slow down due to the mutual attraction of matter.
Predicting existence
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But in 1998, independent teams of scientists, observing Type Ia supernovae, found that some stars are lighter than they should be. And this could only mean one thing: the expansion is not slowing down, but by no means it is accelerating. Later, this was proved based on studies of relic radiation and gravitational lensing. The explanation for this was seen in the presence of something very powerful and capable of counteracting gravity. We call it dark energy, and this name follows only from ignorance of its nature.
Explanation of the essence
There are many theories that express the nature of this puzzle. One of the most plausible ones says that dark matter is evenly distributed throughout the entire universe and interacts very weakly through any channel of fundamental interactions, with the exception of gravity. As mentioned earlier, the universe should stop expanding due to the presence of dark matter and matter, but due to the presence of the so-called negative pressure of the cosmological constant (dark energy), the expansion is accelerating.
There is also a competing opinion that says that there is no dark energy, that these are only unknown properties of gravity. It has the right to exist, but it is extremely unlikely, because the mathematical model of the gravitational field is sufficiently fully developed, and the introduction of some amendments will violate it.
Dark energy makes up 74% of the total energy in the universe. It is opposed by dark matter (be sure to follow the link to understand what dark matter is), but there is much less of it, so it cannot slow down the expansion process.