He is believed to have travelled widely, including India and Ethiopia and his writings mention Babylon and Egypt where Diodorus Siculus says he lived for five years studying their mathematics. Clement of Alexandria records hims as declaring  that among his contemporaries none had made greater journeys, seen more countries, and met more scholars than himself.
Because Leucippus left only a few scraps of writing, but Democritus is comparatively well served with some fifty extant fragments, it is with him that the theory is primarily associated. The problem with atoms, it had been argued, was that between one thing and another there must either be something else - or nothing. This nothing - 'the void' - was a problem, how could philosophers say that 'nothing' existed?
This, however, was precisely what Leucippus did do, and then went on to offer a new kind of explanation of how the world can paper to us via our senses. Democritus explains even that certainly particularly fine and smooth atoms make up our 'souls', and perception arises from the interaction between these and the slightly cruder world atoms.
By convention there is sweet, by convention there is bitterness, by convention hot and cold, by convention color; but in reality there are only atoms and the void 
Part of an early version of this article was taken from the entry on Democritus in 'Essentials of Philosophy and Ethics', edited by Martin Cohen, (Hodder Arnold 2006) and donated to the Citizendium by the author.
- Clement of Alexandria, Stromata, i
- see, for example, http://plato.stanford.edu/entries/democritus/ where it is noted that the contrast here is intended to be that between real and unreal properties