Question: what is the relationship of gravity to the other basic forces of the universe, and what is the dark energy which apparently is causing the expansion of the universe to accelerate?

Paul Brack answered on 13 Jun 2015:

This might be a bit dry, so bear with!
There are four fundamental forces which underpin everything that happens in the universe:
1) The strong nuclear force – very strong, but only acts over a very, very small distance (smaller than an atom); this force keeps the nuclei in atoms stuck together
2) The weak nuclear force – weak, and acts over a slightly longer distance, and is the force that makes radioactivity possible
3) The electromagnetic force – the force that makes magnets stick together or push each other apart, or electrical charges repel or attract each other. Can act over quite long ranges – you can see magnets attracting, for example.
4) The gravitational force – the force that pulls two objects together. Gravity is a bit weird in the world of forces because it only pulls things towards each other; most forces can either pull things together or push them apart, depending on what the things are. Gravity is a very weak force, but it can operate over huge distances – from the Sun to Pluto, for example.
So gravity is one of these 4 fundamental forces – they are called fundamental because they are the foundations – you can’t break down these forces to anything else – and together these 4 forces control everything that happens in the universe.
Or at least we think they do – you also ask about what dark energy is. Some physicists think dark energy is a fifth fundamental force called ‘quintessence’. They reckon it would have about the same strength as gravity. However, this is still quite a controversial idea. The reality is that we really don’t know what dark energy is, we can just see its effects, so we have to believe it is there. Perhaps you can be the physicist who solves it!