Okay, so what about "dark energy"? Well, it's the same kind of story, I'm afraid. As well as the observation that there isn't enough mass in the universe, we have also observed that there isn't enough energy. Basically, we can tell how quickly an object is moving away from us using something called redshift. Redshift is a type of Doppler Effect, but with light. As an object moves away from us, the wavelength of the light that reaches us becomes stretched, and the "redder" it appears (N.B. this could well put it beyond red into infrared, but its still light). Observing the redshift of distant galaxies is how we know that the universe is expanding. However, more recent discoveries have shown us that the expansion of the universe is accelerating faster than we think it should be, and so there must be something that we cannot observe or measure that is making it do that. This is what we call dark energy. Again, dark energy could be energy, or it could be a field or it could just be a property of empty space that we have not yet been able to measure - we just don't know. Basically, the lesson we can take from this is that when scientists call something "dark", it's their equivalent of a shrug emoji.
