PT 64 S3 Q13

Astronomers have found new evidence that the numberof galaxies in the ... ...

Which one of the following, if true, does most to explain why the estimates remain virtually unchanged?
(A) The mass of galaxies is thought to make up only a tiny percentage of the universe’s total mass.
(B) ...
(C) ...
(D) ...
(E) ...

*This question is included in Oct 2011 LSAT (PT 64): Logical Reasoning B