One of the readers sent in the following question:
We answer in order:
The dark stream is a coordinated movement of clusters of galaxies stretched over 3 billion light years, and it does not contradict the known physical laws, although it looks very unusual. However, its existence was determined by only one group of researchers. Independent studies of the same clusters by other researchers have shown that the coordinated motion of galaxies is not observed, which means that there is no so-called «dark stream» either.
Whose research is correct and who was wrong is not yet entirely clear. Therefore, the question of the existence of the Dark Stream remains open. Further research: observations and calculations carried out by independent research teams will establish clarity on this issue.
Dark energy is a special type of energy that causes the accelerated expansion of the universe. The very fact of the accelerated expansion of the universe does not raise any doubts and is confirmed by a huge array of observational data. As soon as the universe is expanding with acceleration, there must be something that is causing this acceleration. This «something» is called dark energy.
Thus, the very existence of dark energy is a scientific fact. At the same time, it is not known what it is, it can be either the theory of gravity with amendments, or some matter. Research shows that the best contender for dark energy is the cosmological constant introduced by Einstein. With it, Einstein’s equations completely describe the accelerated expansion of the Universe, but the physical meaning of this constant and what stands behind it is still unclear.
Dark matter is a special putative type of matter. Dark matter particles do not participate in electromagnetic interactions and cannot be observed. The reason for the hypothesis of dark matter was the problem of hidden mass: galaxies and galaxy clusters behave as if they have much more mass than we can see. This could mean that either our theory of gravity is wrong, or that there is actually more mass there than we see.
The chances of the existence of dark matter, in my opinion, are 90%. No matter how modified the theory of gravity, it does not give the effects that dark matter creates. At the same time, the introduction of dark matter immediately solves the problem of hidden mass and explains many other laws. At the same time, there is always a chance that the correct theory of gravity has not been built yet. In the next 20 years, several large experiments are planned to search for dark matter particles, we will wait for the results.