Hi, I'm new to this forum, and I have a question.

 

OK, matter can be neither created nor destroyed. If that's the case, has the matter which our bodies are made of existed in one form or another for as long as the universe has existed?

To put it another way, when our bodies grow and get bigger, is matter being adding to the universe, or did the matter already exist but is now simply in a new form?

Energy.

To expand a little on a-t-m's reply, it is energy that is conserved, and mass is one form of energy. So the matter or its energy equivalent has been around.

has the matter which our bodies are made of existed in one form or another for as long as the universe has existed?

If by "for as long as the universe has existed" you mean since the Big Bang, then I would say yes.