At the briefest instant following creation all the matter of the universe was concentrated in a very small place, no larger than a grain of mustard.
The matter at this time was very thin, so intangible, that it did not have real substance. It did have, however, a potential to gain substance and form and to become tangible matter.
From the initial concentration of this intangible substance in its minute location, the substance expanded, expanding the universe as it did so.
As the expansion progressed, a change in the substance occurred. This initially thin noncorporeal substance took on the tangible aspects of matter as we know it.
From this initial act of creation, from this etherieally thin pseudosubstance, everything that has existed, or will ever exist, was, is, and will be formed.
Rabbi Moshe ben Nahman (aka "Ramban" or "Nahmanides"), 1194 - 1270; Commentary on the Torah
Wikipedia article here, looked up in reference to a reader's query in the Dail Mail today.