While working on his General Theory of Relativity, Einstein realized that from his calculations it was deduced that the universe was not static but would eventually collapse on itself, due to the gravitational attraction of matter.
This contradicted the beliefs of the time that held that the universe was something static and eternal, so he introduced in his equation the "cosmological constant" that would act as an element that counteracted gravity and kept the universe static.
First Belgian priest and astronomer George Lemaitre and later the discoveries of Edwin Hubble, who before being a telescope was an astronomer, discovered that the universe was expanding.
Based on the Doppler effect and the fact that the light from distant galaxies tended to have a reddish color, Hubble deduced that the galaxies were moving away from each other, making the cosmological constant unnecessary.
Subsequently, once the Big-Bang theory had matured, this departure from galaxies was thought to be due to the expanding force of the outburst, which even today kept galaxies moving away from us.
If so, this would imply that the speed at which the galaxies were moving away would have to lose force and decrease until eventually the force ceased and the universe became static
But more recent studies verified that not only is the universe expanding, but that this expansion is accelerated, that is to say that this expansion is becoming faster and faster, which is beyond logic.
This phenomenon would imply that there is some kind of energy that "pushes" the universe to expand and this energy is what scientists call "dark energy" and that they calculate that it constitutes 70% of our universe.
Although not everyone agrees yet, scientists believe that the vacuum of space would not be as empty as its name suggests and that somehow this energy from the void would be creating new space which would make matter move away.
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