I don't know anything about dark energy, but I can talk about dark matter. So, lets look at the galaxy rotational curve data[1]. (Go look at the picture in th

I don't know anything about dark energy, but I can talk about dark matter. So, lets look at the galaxy rotational curve data[1]. (Go look at the picture in the top right.) Galaxies don't spin like a record, because they aren't stiff. Instead, the stuff spins at different speeds. Also, galaxies are not like the sun and planets, where almost all the mass is at the centre, but rather it's spread out over the disk. Now, we can try and predict how the mass is spread out, based on the size and colour of the stars we can see, and use that to get one prediction of how the galaxy should rotate. We can also measure red/blue shift from stars, and use that to try and actually measure how the galaxy is rotating. The problem is that these two don't match up. Which is a problem. So, we're trying to find ways to explain why they don't match up. Now, one way to do this would be to note that if there was X amount of matter that we didn't account for when we only looked at the stars, then the curve would in fact match up. Since this matter isn't stars, and doesn't give off light, it was called dark matter. And it seems more likely that there's a bunch of mass we can't see than that gravity works weird. [1]: https://en.wikipedia.org/wiki/Galaxy_rotation_curve https://en.wikipedia.org/wiki/Galaxy_rotation_curve