Among the theoretical sciences investigated by Aristotle, natural philosophy and metaphysics are the most universal. He explores these topics in his Physics and Metaphysics. Here, I'll attempt to give a brief summary of some of Aristotle's most important ideas in these areas.
From his predecessors Aristotle takes the idea that there are four basic elements: fire, air, water and earth. But Aristotle develops a new explanation for the distinctions between them. On his view, the physical world is a continuum capable of exhibiting various attributes, and the four elements are determined by two pairs of opposing attributes: wet - dry and hot - cold. Fire is hot and dry whereas air is hot and wet. Water is cold and wet, and earth is cold and dry. A number of observations fit nicely with this theory, particularly the condensation and evaporation of water. The world is further finite and spherical in shape, as it is most notably in the philosophy of Parmenides, and the physical world admits of no vacuum.
While a vacuum is an integral part of atomistic theories as the space between atoms, it is impossible to describe consistently within the framework of Aristotle's theory of substance, which is based on the interdependence of form and matter. On Aristotle's theory, whatever exists is a combination of both form and matter. Matter cannot exist without form nor can form exist without matter. Since a vacuum is defined as a place devoid of matter, a vacuum thus cannot exist. Being devoid of matter, it would necessarily also be devoid of form and hence of any being whatsoever.