One way to look at dark energy is as the 'energy of empty space'. So, when space expands, there is more space, and hence more dark energy.
This extra energy has to be balanced, yes, but it is balanced by a decrease in the curvature of spacetime, which accelerates the expansion of space.
For ordinary matter and dark matter, when space expands, the total amounts of matter stay the same, but it is spread out more. This serves to gradually slow expansion.
For radiation (like light), expansion actually decreases the total amount of energy, which slows the expansion more.
So, in the very early universe, which was radiation dominated, the expansion was slowing, decelerating fairly fast. Later, when matter dominated, the expansion was still slowing, but not as fast. Now, with dark energy dominating, the expansion has started to accelerate. it's all a play-off between the different components of the universe and how the respond to expansion. And that is all about gravity.
This extra energy has to be balanced, yes, but it is balanced by a decrease in the curvature of spacetime, which accelerates the expansion of space.
For ordinary matter and dark matter, when space expands, the total amounts of matter stay the same, but it is spread out more. This serves to gradually slow expansion.
For radiation (like light), expansion actually decreases the total amount of energy, which slows the expansion more.
So, in the very early universe, which was radiation dominated, the expansion was slowing, decelerating fairly fast. Later, when matter dominated, the expansion was still slowing, but not as fast. Now, with dark energy dominating, the expansion has started to accelerate. it's all a play-off between the different components of the universe and how the respond to expansion. And that is all about gravity.
