hi CD, at the instant of the big bang (or the instant our universe came to exist) as said there was no matter, as atoms had not yet formed, the building blocks of the atoms had though. at the first instant of the big bang temperatures would have been in the millions of degrees C and as a consequence the elements of the first atoms could not join to form atoms or matter.  as the universe expanded it coolled and as such the fundamental atomic particles would slow down allowing them to then join into atoms then matter. at that instant of matter creation weight was produced and the speed of light permanently fixed.

re this dark matter, at the moment no one knows just what the extra weight the universe requires to be as it is, DM is the 21st centurys answer to the flat world therory. ie they only see what they can see, not what is actually there.