No, not the Imperium.

But the old Terran Empire, during the Golden Age of Man had such technology, and more.
Mankind at its technological height had technological wonders, and performed acts of science, far beyond the Mechanicus' understanding.

One thing that always ticked me off was the necron antimatter pistols (forgot exactly what they're called). It seems to make sense at first: shoots one particle of antimatter that explodes massively on contact with normal matter. this is something we know can happen with modern science. What I don't understand is how it doesn't instantly explode when leaving the barrel. When on any planet, the atmosphere would be everywhere. Those gasses are still matter, so how does the particle even reach the target and not blow up on contact with everything in between?

I don't know, still bothers me.

It's probably kept within a magnetic field that persists during it's flight to the target, and maybe into the target.
The field wears off, allowing matter and anti-matter to meet.

I believe it is a similar idea to Plasma weapons.

Maybe it has to hit a certain number of particles to slow down enough to cause the reaction, and a gas doesn't have the density to cause the logorithmic energy decrement per collison to make it happen.