Gravity is by far the weakest of the four fundamental forces that exist in nature—the others being the electromagnetic force, and the strong and weak nuclear forces. Yet over large distances it is gravity that dominates. This is because gravity is only ever attractive and because it can never be screened. So while most large objects are electrically neutral, they can never be gravitationally neutral. The gravitational force between objects with mass always acts to pull those objects together, and always increases as they become more massive.
It is thanks to geniuses like Newton and Einstein that we understand gravity at all, yet it is gravity, more than any other force, that continues to present scientists with the most enigmatic of puzzles. To understand why, let's start at the beginning, and consider the historical development of gravity.
The pre-history of gravity
It seems safe to assume that mankind has always known that when we drop an object it will fall downwards. In this sense, we've always been aware of the existence of gravity. It was the cause of this motion that appears to have been the focus of many early thinkers.
Aristotle, who's Physics played a dominant role in European science until as late as the 17th century, based his explanation of gravity on the idea that objects should move towards their natural place in the Universe. Where this place was, he reasoned, should be determined by the composition of the object in question. More specifically, it should depend on how much it contained of each of the four elements: Earth, Water, Air, and Fire.
Aristotle argued that objects composed predominantly from Earth and Water should move towards the centre of the Universe. To him, the centre of the Universe was the place beneath his feet. Objects made from Earth, and that are thrown into the air, must therefore move down towards the ground. Water, he reasoned, is lighter than Earth, as can be verified by dumping some soil into a half-full glass of it. All Water must therefore come to rest on top of the Earth. Similarly, Air is lighter than Water, as bubbles rise in water. The natural place for Air is therefore above Water, and the natural place for Fire is above the Air.