Personally, I lean towards something or other always having existed. 'Stuff' floating about in space so- time too. If time stretches back infinitely, then it seems likely at some point, something would happen.
For my very non scientific brain, the phenomena of black holes seems to fit. So, enough stuff clumping together to create gravitational pull to attract more stuff until the pull is so strong and the matter so condensed that a black hole forms. (Not even sure if that's how they form but, it feels right.) Then, with such a huge amount of matter- enough to create the solar system, under so much pressure- it makes sense it could explode and spew everything back out- the big bang. I have nothing scientific to back it up, it's just something I can picture.
My brain struggles to come to grips with infinity or, something not having a beginning. But, that doesn't mean that they aren't possible. To go from nothing, no matter, no time to everything in this universe feels even weirder. To imagine there being a boundary around the universe also seems weird. I'm really not convinced by a God or being behind it either.