Do you think the explanation for the universe could be something physical or does it have to be metaphysical?
The argument against a materialist cause and a metaphysical cause is a very old one that is much older than Aquinas. The Brahma Sutras addresses the regressus in infinitum against the Vaisesika philosophy (Indian materialist, atomists - https://en.wikipedia.org/wiki/Vaisheshika). In Brahma Sutra 2.1.12 - 2.1.14, (Chapter 2, Section 2 Adhikarana 3, here - https://www.wisdomlib.org/hinduism/book/brahma-sutras):
Adhikarana summary: Refutation of the atomic theory of the Vaiseshikas
Having answered the objection against the Vedantic view, the author of the Sutras now proceeds to refute the Vaiseshika philosophy.
Brahma-Sutra 2.2.12: Sanskrit text and English translation.
उभयथापि न कर्मातस्तदभावः ॥ १२ ॥
ubhayathāpi na karmātastadabhāvaḥ || 12 ||
ubhayathāpi—In either case; na—is not; karma—activity; ataḥ—therefore; tat-abhāvaḥ—negation of that.
- In either case (viz. the Adrishta, the unseen principle, inhering either in the atoms or in the soul) the activity (of the atoms) is not (possible) ; therefore the negation of that (viz. of creation through the combination of atoms).
If the world is created by the combination of atoms, the question is, what causes this combination? If it is a seen cause, it is not possible before the creation of the body. A seen cause can either be an endeavour, or an impact, or the like. Unless there is the connection of the soul with the mind, there can be no endeavour on the part of the soul, according to the Vaiseshika assumption. And since before creation there is no body and therefore no mind, endeavour cannot take place. Similarly with impact etc. If the cause is Adrishta (the unseen principle), does it inhere in the soul or in the atoms? In either case, it cannot be the cause of the first motion of the atoms; for this Adrishta is non-intelligent and so cannot act by itself. If it is inherent in the soul, the soul being then inert, there is no intelligence tc guide this Adrishta. If it is inherent in the atoms, it being always present, a state of dissolution would be impossible, for the atoms will be always active. Again, the soul is without parts like the atoms, and so there is no possibility of any connection between the soul and the atoms. Consequently, if the Adrishta inheres in the soul, it cannot influence the motion of the atoms not connected with the soul. So in all cases original activity in the atoms is not possible, and in the absence of that there can be no combination of atoms, as the Vaiseshikas say. Consequently, the theory that the world is created by the combination of atoms is untenable.
Brahma-Sutra 2.2.13: Sanskrit text and English translation.
समवायाभ्युपगमाच्च साम्याद् अनवस्थितेः ॥ १३ ॥
samavāyābhyupagamācca sāmyād anavasthiteḥ || 13 ||
samavāya-abhyupagamat—Samavaya being admitted; ca—also; sāmyāt—equality of reasoning; anavasthiteḥ—‘regressus in infinitum’ would result.
- (the Vaiseshika theory is untenable) also (because if involves) a regressus in infinitum on similar reasoning, since it accepts Samavaya.
Samavaya or inseparable inherence is one of the seven categories of the Vaiseshikas. They say it is this that connects the dyad with its constituents, the two atoms, since the dyad and the atoms are of different qualities. In that case Samavaya (inherence) itself also being different from these dyads and atoms, which it connects, another Samavaya will be required to connect it with these, and that in its turn will require another Samavaya to connect it with the first Samavaya and so on without an end. Hence the argument would be defective, and consequently the atomic doctrine, which admit Samavaya for combination, is inadmissible.
Brahma-Sutra 2.2.14: Sanskrit text and English translation.
नित्यमेव च भावात् ॥ १४ ॥
nityameva ca bhāvāt || 14 ||
nityam-eva—Permanently; ca—and; bhāvāt—because existing.
- And because of the permanent existence (of the tendency to act or otherwise of the atoms, the atomic theory is inadmissible).
The atomic theory involves another difficulty. If the atoms are by nature active, then creation would be permanent, for dissolution would mean a change in the nature of the atoms, which is impossible. If on the other hand, they are by nature inactive, then dissolution would be permanent, and there will be no creation for the same reason. Their nature cannot be both activity and inactivity, they being contradictory. If they are neither, their activity and inactivity would depend on an efficient cause, like Adrishta, which being always connected with the atoms, they will always be active, and creation would be permanent. If on the other hand, there is no efficient cause, there will be no activity of the atoms and hence no creation. Consequently the atomic theory is again inadmissible.
The best answer that I can give is that of the impossibility of actual infinity. Actual infinity is impossible because allowing it would allow all sorts of absurdities, such as Hilbert's Hotel. Therefore, the infinite regress must stop at some point, at which we have an uncaused cause. If God were to exist, then he would be an uncaused cause, so it is reasonable to assume that this uncaused cause is God unless you can think of a cause that could cause everything without being caused itself - a necessary being.
1/0 = ∞ One (1) is infinitely larger than zero (0) So it is with Existence (something), infinite to ‘non-Existence’ (nothing) In the infinite Existence, the concept of non-existence can exist The concept of existence however cannot exist in ‘non-Existence’ (because it cannot hold or contain anything) Because you exist (the fact you are reading this), proves the falsehood of ‘non-Existence’. (it cannot hold your existence, so itself must be false) And so there is only the Infinite Existence.
What is Infinite Existence? Because of its infinite nature, it can be everything simultaneously.
Is there an afterlife? ‘non-Existence’ is non-existent. Non-existence is only possible as a concept (not absolute) within the Infinite Existence.
Is there a Higher Power (by design)? In the context of Infinite Existence, all things, shapes, ideas exist. The infinite aspect means complete chaos due to the fact that to define a thing or shape, one is faced with infinitely number of other things and shapes to define. There is no pattern, grouping and categorization possible. Now, against the backdrop of complete chaos, when we start observing pattern in our physical Universe i.e. golden ratio (Fibonacci), we can therefore conclude that it is absolutely by design. It is akin to discovering a trail of breadcrumbs in a virgin jungle. In the most fundamental level, the categorization of what is, and what is not is already the evidence of Design.
Metaphorically speaking, see here
I like Thom's answer. But it could be that you're asking a question slightly different than the one he answered. Maybe, for example, you're asking about the kinds of causal "first-ness" that feature in Aquinas's arguments about God as first mover, or first efficient cause (i.e., the first two of his "five ways" here). So just in case, I'll offer another answer from a different angle.
For Aquinas, God is fully actual, and not in any way potential. (See the first argument of the "I answer that" here, where he refers back to the passage I cited above.) That just means (roughly) that he has no capacity for undergoing change. And something with no capacity for undergoing change cannot be caused to be other than what it is (since that's what changing is).
So God cannot be moved or caused by anything. If the regress you're talking about is the regress of movers, then, God stops it because he isn't moved by anything. And if the regress you're talking about is the regress of efficient causes, God stops it because he doesn't have an efficient cause.
(If you want to know why Aquinas thinks God has no capacity for change, his arguments for that are in the second passage I linked above. I'd be happy to add a short explanation of those if you want it.)
If you want an Aquinas-style answer to your lattermost question, it's 'no'. I take it, when you say "universe", you're really asking about the set of all physical things—including a multiverse, if there is one. In that case, if the explanation of the universe were a physical thing, that would mean that the universe is explained by itself or by one of its parts (or members). But for A to explain B is for A to cause B—whether formally, materially, efficiently, or finally.* Properly speaking, nothing causes itself in any of those ways, and parts only cause their wholes materially—which, I assume, isn't what you're asking about.
*Here's a bit on these terms, in case you don't know what I mean.