Does Dark Energy Exist?
Before I talk about dark energy and why that’s an even bigger deal, I need to clarify what we mean by the word “expanding.” The actual observation that you can do in the comfort of your own home (provided you have access to a sufficiently large telescope and a spectrograph) is that galaxies appear to be receding from our own Milky Way. On average, of course: galaxies aren’t simple creatures, and some, like our a-little-too-close-for-comfort neighbor Andromeda, are moving toward us.
This recession is seen in the redshifting of light from those galaxies. The fingerprint frequencies of certain elements are shifted down to lower frequencies, exactly like they are for the Doppler effect.