Why A

Hello @Meredith, You are correct that the addition of 40 billion galaxies would have an impact. However, the word "tiny" tells us that this impact would be very small. If galaxies as a whole are a tiny percentage of the universe's total mass, then multiplying this percentage by 5 is not going to have a significant impact on total mass. A key phrase here is "virtually unchanged." If the stimulus had simply said "unchanged," then I would agree with you. However, virtually leaves room for slight differences.