By law of bivalence there exist only 3 states of existence, Rationally possible/Rationally must/Rationally Impossible.
For a rationally possible case of existence, it's called contingent. (Something which if existed, did not exist prior, can cease to exist or come up in another possible way).
For a rational impossibility it's simply that (Square circles in 2D, a too light and too heavy object..etc).
For a rational necessity it just needs justification for now so one can't define it yet.
Now for the set of all contingencies to exist it needs an explanation.
This explanation can be either a contingent event or a necessary event.
To say that all the set of contingencies came only from contingent explanations, this would yield infinite regression, as these contingent explanations would need an explanation by themselves, and this is perfectly fine if you don't use it for an ultimate explanation of the universe.
This would lead to a fallacy if you wanted to justify "Why there is something rather than nothing?"
Because, even if the universe and existence is infinite and eternal, you showed no explanation anyways.

--Then, this dilemma cannot be explained, except if a first explanation (First cause is necessary in existence; IE It exists uncaused extrinsically).
If the first cause is something contingent then it would need an explanation by its own. If it's a contingent but is caused by its own (breaking contingency definition) this in itself is paradoxical, as this contingent event would need to exist first to make itself come up. So at some point we have the event simultaneously existing and not existing in the same time. For example if a finite universe should cause itself, it would be problematic as at one point it would exist and not exist simultaneously.

--This necessary cause must be the explanation of the set of all other contingent events, so it can't be for the first cause to be inside space or time, as this cause existed prior to space or time.

--This necessary cause needs to be eternal, as FROM (2) this cause is not bound by space and time, and a beginning is absurd when it comes to a-temporal setting. Also it will yield the same paradox if it had a beginning.

--This necessary cause needs to be maximally powerful to be able to be the explanation of the set of all rational possibilities; Because if the necessary cause is not maximally powerful then it was limited, and the first cause would need a prior specifier to limit its nature of existence, and since its the first cause then it cannot be limited as nothing prior to it exists.

--This necessary cause has a freewill; as freewill is the choice between different rational possibilities with no prior cause, and the necessary cause is the first cause so there is no prior causation for it's creation. And given that we only can see one rational possibility of ones self, then the first cause was free and willful to choose this particular possibility to exist rather than any other rational possibility.

--Multiple first causes cannot co exist; As every first cause will not be limited by a prior specifier, so they will all have freewill that may clash together negating each others freewill..