The “Big Bang” is not a sufficient explanation of the origin of the Universe. In other words, it can not be used in order to explain the Universe as we see it. Talking about “dark matter” is just flailing around in the dark.
The first job is to explain where the energy came from that caused the “Universe the size of a pea” to expand to the size of an apple, then to the size of a grapefruit, then to the size of a watermelon, and to its current size.
I would also like to know how it is possible to measure the size of the Universe at its inception, since there can not be an observer standing outside of the Universe with a measuring stick to measure it.
(Obviously it’s not possible.)
Prior to the "Big Bang", there was the "Big Dinner" and a couple of drinks.
One can only go back through causes of causes so far when doing research. Ultimately either there was an original uncaused cause to all reality, or we have an infinite chain of caused states of reality that as a whole is uncaused. The latter is not possible on my view--so like Gottfried Liebniz I take the view there was something that caused everything else that was not itself caused, and understand this to be God, since every set of things one wants to consider in nature seems to be contigent on external causes, so nothing inside nature would suffice.
But the point of the big bang is not to go back to where the energy came from....the Jesuit Priest Geoges Lemaitre who first proposed the theory (calling it a "primordial atom") was just trying to make sense of the implications of Einsteins theories on the universe and predicted it had expanded from a point. Presumably, he thought this was the creative act of God, because, obviously any other explanation would be silly and contrived. And indeed, it took decades for atheists to give up fighting the theory and start contriving their ridiculous alternatives.
But Lemaitre was only doing science, not theology in this case. And that is limited to figuring out how nature works rather than why it is here.
If the Universe expanded from a single point,in the Big Bang theory, and everything is spreading from that point, then surely we must know exactly where in the Universe that point is.
Do we ?