Epicurus believed that the only things that exist are matter and void, and that atoms create everything by falling into space (void). The universe is unlimited and full of empty space that is constantly taken up by these eternal atoms made of up different sizes and shapes. In response to the argument that the universe is limited, he explains that if man were capable to reach the end of the universe, he would be able to stick his fist through the end and wherever his fist reaches, that would be the new end of the universe and this would happen repeatedly. Epicurus states that these atoms can and will sometimes swerve from side to side creating atomic collisions (the swerve) that spontaneously create separate planets and other objects.

Following his belief that the world is made of matter and empty space, Epicurus believed that the world has no beginning, nor does it have an end. It has always existed and will always exist. Nothing comes into existence from nothing, and this implies that there is no higher being that created the universe and man.

Aristotle and Epicurus disagreed on the idea of atomism. Aristotle did not accept the concept that there is the possibility of empty space and void, that there exists a space in which nothing exists. Aristotle disagreed mainly because of his notion of potentiality – wherever there is space, there is potentially a substance. Potentiality is just the possibility of having some form, and what is formed into a substance is matter. Therefore, wherever there is space there has to be matter. Matter never exists unformed; as a result, the idea of empty space, or a void, is a contradiction in terms. Although they both agreed that matter does exist, Aristotle believed that everything is made up of matter, and there is never the chance of void, empty spaces.
(Edited from Aristotle and Epicurus, by Justin P. Wendling on www.rebirthofreason.com).