“The trees that are slow to grow bear the best fruit.” –Molière
So if the Universe is expanding and cooling, what does that mean for the matter in it? Sure, it’s easy to visualize how radiation cools: it has a wavelength, space expands, and so as the wavelength gets stretched, the energy drops.
But the energy must have dropped for matter as well, otherwise it wouldn’t have lost enough kinetic energy to become gravitationally bound into gas clumps, stars and galaxies! And yet, those things very much exist! What’s the resolution to this?