Here is an interesting view of Aquinas. Not necessarily saying that I agree with him though:
"It would seem that the universe of creatures, called the world, had no beginning, but existed from eternity. For everything which begins to exist, is a possible being before it exists: otherwise it would be impossible for it to exist. If therefore the world began to exist, it was a possible being before it began to exist. But possible being is matter, which is in potentiality to existence, which results from a form, and to non-existence, which results from privation of form" (Summa Theologica, First Part, Question 46, Article 1, Objection 1).
Aquinas is saying that possible being is matter. Perhaps Aquinas is referring to things that are in heaven that, as of yet, have not been perceived to all of us humans. Quantum Physics knows that sometimes, subatomic particles act like waves of light and can thus "pass through" other objects. "[De Broglie]...showed how any object [of the subatomic world], of whatever mass, could behave as a wave to some extent" (Adam Hart-Davis et al., The Science Book, p. 229). We know that the bible speaks numerous times of transmutations or metamorphoses of angels, horse-chariots (2 Kings 2:12) and stones, saphire, emerald and etc. that existed before their coming into our physical realm of our 5 senses. Descartes used Galileo's scientific discovery, that the world is in reality colorless, to say that our senses don't perceive the world as it is and are thus doubtful. Muller, a devout catholic, showed how the world is not as we perceive it. "What Muller had shown was that we in fact had no understanding of any outside world beyond ourselves. The external world, as we knew it, resulted from the perceptions of our particular nerve endings: those in the optic nerve, the skin, and so forth. The human mind did not perceive what was going on in an external world, all it knew was the alterations taking place in its nerve endings" (Paul Strathern, A Brief History of Medicine, pp. 207-8). Einstein believed that time is a deception for us humans, and Solomon also states, that God "has also placed ignorance in the human heart so that people cannot discover what God has ordained, from the beginning to the end of their lives" (Eccles 3:11, NET).
"...for us physicists believe the separation between past, present, and future is only an illusion, although a convincing one" - Albert Einstein, in a letter to Besso's family.
Berkhof's belief is that there were two distinctions in the creation of the world, the first is God's active creation and denotes an act of God whereas the second is passive creation and denotes "[the] result, the world's being created". He says that we have no right to draw creation as an act of God (the first creation) into the temporal sphere:
"God’s eternity is no indefinitely extended time, but something essentially different, of which we can form no conception. His is a timeless existence, an eternal presence. The hoary past and the most distant future are both present to Him. He acts in all His works, and therefore also in creation, as the Eternal One, and we have no right to draw creation as an act of God into the temporal sphere. In a certain sense this can be called an eternal act, but only in the sense in which all the acts of God are eternal. They are all as acts of God, works that are done in eternity. However, it is not eternal in the same sense as the generation of the Son, for this is an immanent act of God in the absolute sense of the word, while creation results in a temporal existence and thus terminates in time. Theologians generally distinguish between active and passive creation, the former denoting creation as an act of God, and the latter, its result, the world’s being created. The former is not, but the latter is, marked by temporal succession, and this temporal succession reflects the order determined in the decree of God. As to the objection that a creation in time implies a change in God, Wollebius remarks that 'creation is not the Creator’s but the creature’s passage from potentiality to actuality.'" (Systematic Theology, Part One, III, C, 3, d).
My belief? I have yet to form my belief I suppose.