It's generally agree that the universe started with the "Big bang"; from a dense collection of matter/energy that expanded and eventually became what we now see. There's considerable evidence for this, most notably the microwave background radiation.
But where did this come from? How could something come from nothing?
First, let me explain about non-questions. It would be nice if every question had an answer; certainly grandson.1 has learned to ask "why?" repeatedly, and that's good. But there are some questions that simply don't make sense. A non-question is something that sounds like a question, has the
grammatical form of a question, but is actually nonsense; for example "What is the smell of the fourth side of a triangle"
and "what is the colour of happiness". A triangle, by definitiion, has three sides. If it has four, it isn't a triangle. And a triangle, being an ideal thing, doesn't have a smell, unless you make a physical triangle and rub garlic (or something) on it, in which case it can have any smell you want, and the question still doesn't make sense.
How could something come from nothing? In one sense, that's a non-question, similar to asking "what is north of the north pole". There cannot be anything north of the north pole, because using our definition of north, that's as north as you can get. The earth is spherical (roughly) and that curvature means that we can have a concept (north) whereby asking "what is north of the north pole" is a non-question.
Similarly, the universe, although in more dimensions. So perhaps "what happened before the big bang", is another non-question.
How could something come from nothing? Well, I don't know. I've never seen nothing, I don't know of any example of nothing, and I don't think anyone else does either. So we don't know what are the properties of nothing. We can only guess. So here's a rather pretty guess.
Maybe one of the properties of nothing is that it's intolerant. Nothing doesn't exist, so maybe nothing *cannot* exist. Maybe if nothing were to exist, then it would immediately, intolerantly, turn into something.
And here we are.