What cosmological problem is “dark energy” believed to solve?

The theory behind the expansion of the universe was that it would gradually slow down as gravity pulled everything closer together. However, later observations revealed that the rate of expansion had increased rather than decreased, as would have been predicted theoretically. This discovery led to the discovery of "Dark Energy," which is the name given to the solution to the problem.

According to my understanding, the acceleration of the universe's expansion must be explained by something they've called "dark energy".

The universe was expanding, as astronomer Edwin Hubble found when he observed the shifting of light from distant galaxies toward the red end of the spectrum, which could only be explained if those galaxies were rapidly vanishing from our galaxy. He also found that the faster a galaxy was vanishing, the further away it was from us.

In recent times, it was found that not only was the universe expanding, but that it was expanding at an increasing rate as well. This seemed to be in stark contrast to our understanding of gravity, which states that the rate of expansion should be decreasing rather than increasing.

Consequently, it was concluded that dark energy existed and that it operated as an antigravity force across these enormous cosmological distances.

Dark energy provides a theoretical explanation for this acceleration, suggesting the presence of an unknown form of energy that permeates all of space and exerts a repulsive force, counteracting gravity on cosmological scales. Dark energy is thought to solve the problem of the universe's accelerated expansion. Supernovae and other observations show that the universe is expanding at an accelerating rate rather than slowing down as would be expected from gravitational attraction alone.