(Not sure if anyone has asked this before) This idea might be a little vague and I'm sure there is a simple solution but its been annoying me since I thought of it and i'd like to know the answer. If you convert energy into matter then surely matter (with mass) exerts a gravitational pull on other matter (with mass) around it. So if energy does not exert a gravitational pull - and I assume it doesn't - then surely by changing energy to matter you create gravitational potential energy. But you can't create energy from nothing says the first law of thermodynamics, so how does this work? Help appreciated.