There’s an intuitive principle known as the principle of sufficient reason, which that says everything that exists has an explanation for it’s existence. This isn’t the same thing as saying that everything which exists has a cause of its existence. But this principle, the principle of sufficient reason, states that every THING has an explanation either in the necessity of its own nature or an external cause. The former is a necessary being, the latter is a contingent being. A contingent being is a being that could fail to exists, and these are most beings that we are acquainted with. Beings that could fail to exist include things like water bottles, paper, lights, books, flowers, trees, humans, planets, universes, windows, stop signs, angels, televisions, golf clubs, etc.
Now when it comes to necessary beings, there aren’t as many examples; hence, the concept of necessary being might be a little more vague. A necessary being is a being that couldn’t fail to exist. If it exists, it exists necessarily, and there’s no way it could fail to exist. Some examples of necessary beings include abstract objects like numbers (e.g. the number 2 or any other number) and abstract objects like properties (e.g. the property of goodness or red). Necessary beings also include beings like God. If God exists, then he necessarily exists; he couldn’t fail to exist. This weaker form of the principle of sufficient reason doesn’t state that every fact/truth has an explanation for its existence; rather, the principle states that every being or thing that exists has an explanation.
With contingent beings it’s obvious that these things have an external explanation, which means that the cause comes from something outside it. For example, the cause of me was my parents. However, the situation is different with necessary beings. With necessary beings, they don’t derive their being from other beings. This doesn’t mean that they literally caused themselves to exist, because that would be impossible. For to cause yourself to exist, you already have to exist! It would be helpful to think about it more in terms of a being that is uncaused. So if God existed, he would be uncaused. But being a necessary being means more than being uncaused. Remember, necessary being is a being that couldn’t fail to exist. Obviously an uncaused being could pass out of existence or not have existed at all. Hence, being uncaused is at least a part of what it means to be necessary, but it isn’t identical to being a necessary being.
It seems like the universe could not have existed, even if it was eternal. Because even if it was eternal, we could still ask why the eternal universe exists rather than nothing, and we could still ask this question even if there was an eternal multiverse. But when we get to God or numbers, asking such a question doesn’t make sense, because these things by definition couldn’t fail to exist, if they exist.
Hence, if the universe does have an explanation, that explanation would be a being that is timeless, spaceless, immaterial, uncaused, and necessary being. There are two options here. Either an abstract object caused the universe to exist or God. But abstract objects couldn’t have created the universe because abstract objects can’t cause anything by definition. An abstract object by definition is casually impotent.
Now the term “explanation” is used in the formal argument instead of just “cause” because “cause” can be a little confusing. For example, I referenced that existing by a necessity of one’s nature doesn’t mean that said being brings itself into existence. Hence, we can use the phrase “cause” to mean something like, “An answer to a why question”. Here the question is “Why is there something rather than nothing”. The claim is that if everything was contingent, this includes a states of affairs where there are an infinite number of uncaused-contingent beings, then we are left with the question of why there is anything at all.
When we come to the term “cause” we usually think about it in terms of things that precede their effects. For example, the effect is that I have cancer, the cause was smoking. The smoking occurred prior to me getting cancer. But there’s also such a thing as simultaneous causation. Simultaneous causation is the cause and effect happening at the same instance. The brick hitting and the window breaking, can be seen as an instance of simultaneous causation; the brick hitting and window breaking occur simultaneous, such that we can also look at he situation from two aspects.
Some will object to the whole principle by saying that there’s plausibly no such thing as a necessary being, it’s an absurdity. But why is it absurd? There’s no logical contradiction in the idea of a metaphysically necessary being. Think about it. Is there a contradiction in the notion of a being that couldn’t fail to exist? Obviously not! And the principle doesn’t state that there are in fact necessary beings. The principle states that if there are necessary beings, the explanation of their existence is found in their own nature.
Another objection is that every contingent being needs an explanation, yes; however, this is only true of everything in the universe and not the whole universe itself. But why think that? Isn’t that arbitrary? Couldn’t we still ask why the whole universe exists rather than nothing? It seems that we could. If we blow up the size of a ball to the size of a planet, we realize it still needs an explanation, if we blow it up to size of the milky way galaxy, then we have the same problem. And if we blow it up to be the size of the universe, then we still have the exact same problem. Why would it just be universes that don’t require an explanation of their existence? What is it about universes? Even if we’ve explain everything inside the universe by reference to everything else, we can still ask, “But why is there anything at all?”