Atheists: If there is no God, then how did the universe begin?

If you say "there has to be a grand designer" then you are supposing there must be a super grand designer that designed the grand designer. And so on. Design arguments always fail because of this problem. The Universe 'As we know it' began after the Big Bang. What was before that is at present unknowable. It would require a new physics, new mathematics, & new computer technology to ever discover what preceded the Big Bang.