“In order for the light to shine so brightly, the darkness must be present.” -Francis Bacon
If you had previously thought the Universe contained a certain amount of stuff, like stars, galaxies and matter, then you might think that learning it had ten times as many galaxies might mean it had ten times as much matter. And if this were the case for matter like us, made of protons, neutrons and electrons, perhaps there wouldn’t be a need for something like dark matter, after all.
But if we want to examine this claim, we need to look at two things: what the motivation for dark matter is in the first place, and what the observations of additional galaxies actually tell us. The biggest takeaway is that there are three independent observations all pointing towards the fact that only 5% of the energy density of the Universe is made up of normal matter, and even if the Universe were to have ten trillion galaxies, that would still fit comfortably within that 5%.