Robert Adler has a nice article with the title Why is there something rather than nothing?explaining what our best theories say about why the universe exists at all and why it has the properties it does, such as being so flat and uniform.
He starts out with the idea that according to quantum field theory, a vacuum is not nothing in the classical sense but consists of quantum fields and is unstable. At small scales it consists of a foam of particle-antiparticle creation and annihilation and that in some cases inflation causes one of the bubbles to expand to macroscopic scales, creating all the matter we see in our universe and that as long as the universe if flat, the positive energy required for matter creation is exactly balanced by negative gravitational energy. Thus the creation of the universe does not require a net energy creation.
This explanation seems to presume that the existence of relativistic quantum fields even in a vacuum is a given and some have asked why that should be the case, and that a true ‘nothing’ would not contain these fields at all. In that case, the question raised by Adler shifts from how we get the material universe from a vacuum containing only quantum fields to how we get a vacuum of quantum fields from a vacuum of nothing. Whether this is a meaningful question is a contentious issue that I addressed last year in my post Much ado about ‘nothing’.