Something that fascinates me is the idea of scale of our universe. In particular, “scale” relative to a more traditional size/distance scale that we use more often. The metric scale, whose utility to humans generally ranges from millimeters to kilometers (measurements that we are easily able to estimate and compare), represents an indescribably minuscule spectrum of distance measurements when compared to the gargantuan size of our universe. See this slide show depicting the scale of our universe.
The universe is made of superclusters, which house local groups, one of which containing the Milky Way galaxy, which contains our solar system–inside that solar system lies our planet, Earth. Even the small minutiae of the universe (planets like Earth) are smaller in comparison to the Universe than the cells are that make up a human body. The diameter of the Earth is a breathtaking 12,742km–a number still much greater than most are able to comprehend–but the Earth itself contributes negligible mass to the universe as a whole. This subject is particularly intriguing to me because it challenges me to think in terms of distances/sizes that are much larger than we can comprehend.