What is Stephen Hawkings explanation for the origin of the universe?

He hasn't got one, just the same as every other physicist including me. We can only speculate . Actually knowing is going to be impossible. Oh and the Big Bang is not about the origin only the evolution of the Universe.

Buy his book "A Brief History of Time" and I promise you won't be disappointed. His explanations (like all physicists) is highly detailed and can't be brought to justice via YA.

To understand it, you must understand Einstein's theory of Relativity and have some sort of an idea of Quantum mechanics. This allows you to understand the concept of 'nothing' and what they mean by the 'singularity'.

Hawking correlated the beginning of the universe with what happens inside a black hole. Essentially, the mathematics go all hay-wire and show a point of infinite density where time 'stops'. This is called a singularity. However, in terms of the big bang, think of the singularity as a moment in time. The actual beginning. Due to observation and mathematics, it seems at one time the universe was in a state of infinite density where time 'stopped' (much like what happens inside a black hole according to the math).

It is still unknown what the 'cause' of the Big Bang was, but Hawking sort of developed the idea of the singularity at the beginning of the universe.

Note: When a physicist says that the universe came from nothing, be sure you understand what they mean by 'nothing' (especially in terms of time and space). Learn why they came to this conclusion (relativity). And learn truly why physicists discovered the 'singularity' (yes, discovered within the mathematics (not fully understood however)).

Bigger Note: If you don't know yet, learn the scientific method and what a Scientific Theory is. Hint: it is not a guess, it is nearly fact with sometimes 99% certainty. It is often shown accurate through thousands upon thousands of experiments and not a single one has shown otherwise (relativity and quantum theory).

Hawking was a fan of the Big Bang Theory which has can technically branch off into 7 different cosmological models, but all of them are essentially the same with differences varying in exact amounts or times. Regardless, the Big Bang Theory has made remarkable predictions and is backed by a substantial amount of testable and observable evidence.

Final Note: The Big Bang Theory doesn't necessarily state what 'caused' the big bang. Ideas vary from 'no cause' to 'cyclical process between big crunch and big bang' to 'multi-verses colliding' to 'God did it'.

Professor Hawking theorizes how the universe formed, not why.

His is the same as everyone else's: the Big Bang model.