The theory is primarily based on the Theory of Relativity, developed by Albert Einstein in 1916. As Joversity notes, it was assumed and believed that the universe is static and eternal. However, the theory of relativity changed this perception. It suggested that the universe is not static and usually ‘bends’ in the presence of matter. At the same time, Einstein sought to explain this theory and its practicality by help of the Gravitational Theory, which shows that if the universe had stars and planets placed arbitrarily at a distance from each other, they would end up attracting each other in clusters. Since this is not the case, the universe must also have a repulsive force among galaxies which gradually increases in strength with the expansion of the universe, thus ensuring that galaxies and all within them stay in place. He also came up with the formula that explained the curvature of the universe (which encompasses the idea that time can bend and is subject to velocity over the universal expanse). In this equation, Curvature=Energy-momentum. This shows that the curvature of the universe depends on the energy and momentum of the universe. In the same equation, Einstein subtracted the Cosmological Term from the Curvature, i.e, curvature -cosmological term=energy/momentum.
Einstein further observed that the universe is expanding, an established fact today. Therefore, with the insignificance of the little repulsive force he had ‘discovered’, he dropped it, opting to see the effect that gravity would have cancelled out the expanding universe. This also raised the ‘big bang theory’ which stipulates that the earth might end due to overpowering of the expansion by gravity or the reverse, where the expansion overpowers gravity (http://www.youtube.com/watch?v=2keSEOtOyuQ).
The expansion has been examined to occur with the velocity that is proportional to the distance of objects around the earth, i.e, a star that is three miles away moves further three times faster, while one two miles away moves away two times faster. Thus, the universe is expanding uniformly in all directions.
Using the analogy of a moving train, Jovarsity explains how we know that the universe is expanding. As stars and galaxies move further from the earth, their (red) light wavelength increases (red-shifted), thus leading to the conclusion that they are truly moving away/expanding.
It is easy to determine the velocity of expansion of the universe, but not the distance, since it requires a standard unit for measurement. This used to be a problem, until the discovery of exploding stars, which let out light brightness equivalent to ten billion stars. The exploding stars are the ‘standard candles’ used in the measuring of the distance of the objects expanding, thus used to determine the rate of expansion of the universe. By this measure, the expansion rate is at 10% (ten percent).
In view of the Cosmological term, once Einstein got rid of it, it was mathematically incorrect just to scrap it off, though geometrically and reasonably correct. Therefore, once it is moved to the other side, it implies that curvature=energy/momentum + cosmological term. It implies that it is a contribution to the universal energy. Since this is impossible, the term has been described as nothing. This ‘nothing’ has been described as millions of virtual particles that bombard, pop in and out of the empty space within such a little, immeasurable amount of time, thus momentarily (though definitely not visibly) forming the weight of the ‘nothing’. It has been discovered through research and practicals that the ’nothing’ separating the spaces between the protons, neutrons and atoms of a particle contribute 90% (ninety percent) of the weight of the particles, even the least of them.
As a result, it turns out that empty space does have mass/ weight and at the same time, energy (from the popping and disappearing particles) . By calculation, the ‘nothing’ of the universe is a gazillion times energy and magnitude than our known universe.The above implies that the energy of the ‘nothing’ far outweighs the energy of stars, planets, particles and objects within the universe.Since this is almost impossible, practically, the weight must be zero.