The universe has apparently always existed in some form. It exists and, like arguments for an even more complex "given" (God himself) simply "is".
That "is" could be a chaos of atoms in a swarm of attraction and repulsion or it could be something else. I don't know and neither do you or any mastermind elsewhere. But, through countless eons that swarm of existing "stuff" coalesced in various ways.
That is all we really have to go on. If it doesn't sound pretty or doesn't appeal to anybody personally---well, climb into your head and make something better up that warms the fuzzies of your mind! Everybody else has!!
We have to be very tidy and even fastidious when we ask an important question.
Why?
We can end up with a Trojan Horse question that has smuggled hidden concepts into the chain of logic. The end results are then skewed beyond repair.
Existence exists.
A=A
If we ask "why?" DOES existence exist.....that is different from asking: Why "should?" existence exist.
All of man's (actual) knowledge about (actual) things comes from his senses. His ability to focus and analyze his percepts vary from individual to individual. But, collectively man's data is sensory (for better or for worse). This data by itself isn't perfectly understood. That is why he tests it variously.
The second means of man's knowledge is by inference. He juggles, blends and abstracts by mixing the __actual__with the__imagined. The results can be theoretical guesses that are worthless or masterpieces of insight into how the universe operates.
Having said all that...let's face facts. We don't have any sensory data about the origin of existing things beyond the observable cycles. We can project forward and backward in time the chain of causes and effects, but, at some point it all breaks down in the built-in chaos of unpredictable variance.
There is plenty hearsay and plain old word-of-mouth handed down through the ages. "Answers" purporting to be from an "outside source" (god) proliferate through the world. But, that too is beyond actual sensory limits.
So, vast numbers of primitive people (and even sophisticated moderns) go ahead anyway and outdistanced proof and fact-checking. They just shrug and accept whatever appeals to them by creating an "otherness" to existence which is beyond anybody's perceptions except the wishful thinking of imagination.