Astronomy Picture of the Day . ... Oxford U. Explanation: Our universe is filled with galaxies. Galaxies -- huge conglomerations of stars, gas , dust -- and mysterious dark matter are the basic building blocks of the large-scale universe . Although distant galaxies move away from each other as the universe expands , gravity attracts neighboring galaxies to each other, forming galaxy groups , clusters of galaxies , and even larger expansive filaments. ... Publications with keywords: galaxies . ...