Does Lawrence Krauss theory say that the total energy in the universe is equal to zero or that the energy in the entire universe remains constant and the mass in the entire universe also remains constant, meaning that if mass is converted into energy, it must be converted somewhere energy into mass. I don't know what's right
His theory says that the sum of all energies is equal to zero. Positive energy in the form of matter is exactly cancelled out by negative energy in the form of gravitational energy. More on the Wikipedia page about so called zero-energy universe. It is worth noting that his theory is supported by two observations in 1998, but many perceive it as some sort of philosophical speculation.