Physicists have observed the cosmic radiation background to conclude that the universe is flat (or within the margin of error of being flat). This means that the Universe contains the critical density needed to keep it flat, which is a mix of ordinary matter, dark matter and dark energy (I would imagine that other particles such as photons don't contribute to this density as they are massless). The discovery of this "dark energy", responsible for accelerating the Universe at an increasing rate, brings the critical density from about 0.3 to 1.0. Through the equation E = mc2, we can see that empty space really has "mass" that curves spacetime. However, dark energy has the same density. So, as the Universe expands and its "volume" increases, there is more dark energy. Since E = mc2, wouldn't that mean that as the universe expands, its mass expands. And wouldn't that mean that: The Universe is gaining energy? But in order for the universe to be flat, there must be a net energy of 0 (or close enough to it, for any discrepancy to be explained by quantum fluctuations -and possible inflation). This means that the positive energy possessed by matter is cancelled out by the negative energy of gravity. However: If the Universe is expanding, and objects are getting further and further apart, that would mean that gravitational negative energy is decreasing while there's more dark energy to push it further apart. Wouldn't that imply that the positive accelerating energy of dark energy is increasing, while the negative energy of gravity is decreasing? And: Is there therefore an increasing net positive amount of energy in the Universe (implying that energy is not conserved globally)? Maybe the universe was just flat at the time of inflation (which we have observed to be flat), and its becoming more and more saddle shaped over time. Maybe the universe just started out as "net zero energy" but this changed once expansion started. Or maybe, as more dark energy is being produced through expansion, it's creating more "negative energy mass" proportional to the amount of positive acceleration that it is producing? However, if dark energy's "gravitational pull" is cancelling out its "positive expansion", then why is it still accelerating regardless. Thanks for your replies!