The first step in that long quest is to determine where dark energy is and where it’s been. Astronomers can do that by measuring the location and redshift of millions of galaxies, thereby mapping the universe’s expansion. As galaxies move away from Earth, the light they emit becomes stretched in the direction of the “red” end of the visible light spectrum. The more distant a star, the farther light emitted by that star has traveled through an expanding universe, and thus the more redshifted it will be. If the galaxies in one part of the sky are accelerating away from us more quickly than those in other places, then that part of the sky contains more dark energy.