What was the density of matter at 10^-35 ATB?

Answer 1

If you mean 10^-35 seconds ATB, that would put the universe in the so-called inflationary epoch. In that relative short time frame (way less than a second span), the universe itself is thought to have expanded faster than the speed of light. While the universe continues to expand, the expansion during the inflationary epoch accounts for almost all the volume of the universe as we know it today. Therefore, given all the mass-energy of the universe at the big bang is the same mass-energy of the universe today, and all the volume of the universe during the infaltionary epoch (the time frame you're asking about), the density of the universe back then is pretty much the same as it is today.

Why? Because M/V = rho (density) and M, the mass-energy, is constant and V during the infaltionary epoch is about the same as V, the volume, is today.

Oh, if "faster than the speed of light" bothers you, remember mass-energy is limited to that speed, usually denoted as c. But during the inflationary epoch, it is the universe itself that is expanding, not mass-energy; so there is no such limit on the boundaries of the universe.