The following written answer from Cornell University may be able to ask your answer best
http://curious.astro.cornell.edu/about-us/101-the-universe/cosmology-and-the-big-bang/general-questions/570-where-did-the-matter-in-the-universe-come-from-intermediate
I hope this may help you !

The universe is expanding, yes, but more matter is not being created to fill in the gaps.  As a consequence of this, the universe's density decreases over time.

The idea that more matter had to be created with the universe's expansion, at such a rate as to keep the overall density from changing, was popular in the 50's and 60's.  It was known as the Steady State theory.  Observations in the years that followed proved inconsistent with this model, favoring the Big Bang model--a universe that was extremely hot and dense in the past, and has been expanding, cooling and diluting ever since--instead.