Our universe is around 13.8 billion years old. Over the vastness of this time, the tiniest of initial asymmetries have grown into the large-scale structures we can see through our telescopes in the night sky: galaxies like our own Milky Way, clusters of galaxies, and even larger aggregations of matter or filaments of gas and dust. How quickly this growth takes place depends, at least in today's universe, on a sort of wrestling match between natural forces: Can dark matter, which holds everything together through its gravity and attracts additional matter, hold its own against dark energy, which pushes the universe ever further apart?
