Astronomers have found new evidence that the number of galaxies in the universe is not 10 billion, as previously believed, but 50 billion. This discovery will have an important effect on theories about how galaxies are formed. But even though astronomers now believe 40 billion more galaxies exist, many astronomers' estimates of the universe' s total mass remain virtually unchanged. Which one of the following, if true, does most to explain why the estimates remain virtually unchanged? ['The overwhelming majority of galaxies are so far from Earth that their mass can be only roughly estimated.', 'The number of galaxies that astronomers believe exist tends to grow as the instruments used to detect galaxies become more sophisticated.', "The mass of galaxies is thought to make up only a tiny percentage of the universe's total mass.", "There is no consensus among astronomers on the proper procedures for estimating the universe's total mass."]