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

In the beginning it was thought that the expansion of universe will gradually slow down as the gravitational force will pull everything closer . But later the observations showed that the rate of expansion has actually gone up instead of going down as expected theoretically . The solution for this problem was named - " Dark Energy " .
For more detailed information , I strongly recommend visiting this link :
https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy

The way I understand it, some entity they have labeled "dark energy" is required to explain the acceleration in the expansion of the universe.

Astronomer Edwin Hubble discovered that the universe was expanding. He made this connection by observing the shifting (towards the red end of the spectrum) of light coming from distant galaxies. This could only be explained if those distant galaxies were receding from us at great speed. Furthermore, he established that the further away the galaxy was, the faster it was receding.

Fast forward to recent years, when it was discovered that, not only was the universe expanding, but that the rate of expansion was increasing. This seemed profoundly at variance with what we know about gravity - the rate of expansion should be decreasing, not increasing.

Therefore, the existence of dark energy, working as an anti-gravity force as these great cosmological distances, was deduced.