Well, the universe is. The fluctuations in this image, even though they look prevalent, are very tiny, so tiny in fact that we can say that energy density is constant barring quantum fluctuations. This is what people mean when they say "the universe is flat." This image is of light from the universe when it was about 300,000 years old, and so it would be impossible for energy density to be so constant over such distances, as information cannot travel fast than the speed of light. From this, we can infer that there is infinite energy, and therefor the universe is infinite.