Gravity is not radiation. The expansion of the universe does work against gravity but there are limits. Gravity decreases with distance. So if the universe is expanding fast enough (it is) it will overcome gravity. Imagine setting a bomb off between two asteroids. Use a small bomb and they move apart but eventually drift back. Gravity wins. But use a big enough bomb and they both hit escape velocity. Gravity looses. The objects are moving away from each other fast enough that the force of gravity falls away so fast that it will never turn them around let alone bring them back together.

Dark energy is the concept that while the universe is expanding, gravity wins / everything is at escape velocity, it shouldn't be expanding faster. Dark energy is the fact that the two asteroids separated by the bomb (the big bang) aren't just at escape velocity, something is causing them to accelerate.