Why is matter not being created at the present time, nor being destroyed?
Matter is being created and destroyed now. For example, a high energy X-ray can collide with the nucleus of an atom and disappear and two particles, an electron and an anti-electron (a.k.a. positron), will appear in its place. So extra matter is being produced from no matter. The important thing is that the amount of total energy stays the same, but the energy can change its form from electromagnetic radiation (the X-ray) to matter (the electron and positron). Also, an electron and positron can collide with and annihilate each other, producing X-rays.
Mac Mestayer, Staff Scientist (Other answers by Mac Mestayer)