It is still debated if he believed it or not; he doesn’t write it like it is thought-experiment:

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“The new concept of the universe. The universe exists; it is nothing that grows into existence and that passes out of existence. Or, better still, it develops, it passes away, but it never began to develop, and has never ceased from passing away; it maintains itself in both states. It lives on itself, its excrements are its nourishment. We need not concern ourselves for one instant with the hypothesis of a created world. The concept create is to-day utterly indefinable and unrealisable; it is but a word which hails from superstitious ages, nothing can be explained with a word. The last attempt that was made to conceive of a world that began occurred quite recently, in many cases with the help of logical reasoning,—generally, too, as you will guess, with an ulterior theological motive. Several attempts have been made lately to show that the concept that "the universe has an infinite past (regressus in infinitum) is contradictory, it was even demonstrated, it is true, at the price of confounding the head with the tail. Nothing can prevent me from calculating backwards from this moment of time, and of saying: "I shall never reach the end"; just as I can calculate without end in a forward direction, from the same moment. It is only when I wish to commit the error—I shall be careful to avoid it—of reconciling this correct concept of a regressus in infinitum with the absolutely unrealisable concept of a finite progressus up to the present; only when I consider the direction (forwards or backwards) as logically indifferent, that I take hold of the head—this very moment—and think I hold the tail: this pleasure I leave to you, Mr. Dühring!...

I have come across this thought in other thinkers before me, and every time I found that it was determined by other ulterior motives (chiefly theological, in favour of a creator spiritus). If the universe were in any way able to congeal, to dry up, to perish; or if it were capable of attaining to a state of equilibrium; or if it had any kind of goal at all which a long lapse of time, immutability, and finality reserved for it (in short, to speak metaphysically, if becoming could resolve itself into being or into nonentity), this state ought already to have been reached. But it has not been reached: it therefore follows....

This is the only certainty we can grasp, which can serve as a corrective to a host of cosmic hypotheses possible in themselves. If, for instance, materialism cannot consistently escape the conclusion of a finite state, which William Thomson has traced out for it, then materialism is thereby refuted. [Pg 428] If the universe may be conceived as a definite quantity of energy, as a definite number of centres of energy,—and every other concept remains indefinite and therefore useless,—it follows therefrom that the universe must go through a calculable number of combinations in the great game of chance which constitutes its existence. In infinity, at some moment or other, every possible combination must once have been realised; not only this, but it must have been realised an infinite number of times. And inasmuch as between every one of these combinations and its next recurrence every other possible combination would necessarily have been undergone, and since every one of these combinations would determine the whole series in the same order, a circular movement of absolutely identical series is thus demonstrated: the universe is thus shown to be a circular movement which has already repeated itself an infinite number of times, and which plays its game for all eternity.—This conception is not simply materialistic; for if it were this, it would not involve an infinite recurrence of identical cases, but a finite state. Owing to the fact that the universe has not reached this finite state, materialism shows itself to be but an imperfect and provisional hypothesis.” 1067.

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“And do ye know what "the universe" is to my mind? Shall I show it to you in my mirror? This universe is a monster of energy, without beginning or end; a fixed and brazen quantity o; energy which grows neither bigger nor smaller, which does not consume itself, but only alters its face; as a whole its bulk is immutable, it is a household without either losses or gains, but likewise without increase and without sources of revenue, surrounded by nonentity as by a frontier, it is nothing vague or wasteful, it does not stretch into infinity; but it is a definite quantum of energy located in limited space, and not in space which would be anywhere empty.

It is rather energy everywhere, the play of forces and force-waves, at the same time one and many, agglomerating here and diminishing there, a sea of forces storming and raging in itself, for ever changing, for ever rolling back over in calculable ages to recurrence, with an ebb and flow of its forms, producing the most complicated things out of the most simple structures; producing the most ardent, most savage, and most contradictory things out of the quietest, most rigid, and most frozen material, and then returning from multifariousness to uniformity, from the play of contradictions back into the delight of consonance, saying yea unto itself, even in this homogeneity of its courses and ages; for ever blessing itself as something which recurs for all eternity,—a becoming which knows not satiety, or disgust, or weariness:—this, my Dionysian world of eternal self-creation, of eternal self-destruction, this mysterious world of twofold voluptuousness; this, my "Beyond Good and Evil" without aim, unless there is an aim in the bliss of the circle, without will, unless a ring must by nature keep goodwill to itself,—would you have a name for my world? A solution of all your riddles? Do ye also want a light, ye most concealed, strongest and most undaunted men of the blackest midnight?—This world is the Will to Power—and nothing else!And even ye yourselves are this will to power—and nothing besides!”