The official reason for it is the observation of so called Hubble's redshift that has been discovered by Edwin P. Hubble around 1929 and claimed ever since by scientific officials to be a Doppler effect caused by the galactics moving away from us and therefore being an evidence that the unverse is expanding.

This hypothesis has a bunch of problems.

The first and the most important problem is that this hypothesis requires that the principle of conservation of energy is not valid (for which there is no evidence whatsoever). It is nonplusing most physicists but none of science officials. They just say: "so what, so it happens that matter can be created from nothing, what's wrong with that?" They ain't physicists so they don't know what's wrong. And usually they are too old to learn.

The second problem is that Hubble's redshift doesn't need to be Doppler effect and most likely it ain't. Einstainian gravity is able to explain this effect quite well as a metric redshift under condition that the principle of conservation of energy is valid. And this explanation predicts Hubble's redshift exactly as it is observed. With all its peculiarities (like that the apparent expansion should look as if it were accelerating). "But wait a while" say scientific officials "we just proved that the principle of conservation of energy ain't valid!" Apparently they didn't learn not only any physics but no logic neither. Such "proof" is forbiden by logic and the simple logic error commited here is called tautology or circular reasoning.

The third problem is that as it was mentioned above Hubble's redshift looks as if the expansion of the universe were accelerating and official science doesn't allow such expansion, so as it invented creation of matter from nothing to explain Hubble's redshift it has to invent dark energy to explain why this expansion is accelerating. Needless to say that nobody knows what "dark energy" is. "So what?" say scientific officials "it might be something which was not discovered yet because it does not exists on the earth. And what's wrong with that?"

One might say that since there exists a simple explanation involving known things only, why do we need all this exotic stuff (which is called Occam's Razor, an idea of one William from Occam). But usually one says nothing because scientific officials except being experts in cosmology also control all the money in science, and for a scientist to be barred from doing science for life is not a very desirable thing. So we have the Big Bang hypothesis for well over 50 years and not many people dare to protest except some astronomers who say that "something has to be done with it since otherwise all the astronomers look like idiots".