Life on smaller, faster scales has been posited, but not in anywhere we can currently see. See, for instance, Dragon's Egg [1], which is a science fiction novel that very seriously tries to discuss life on the surface of a neutron star. I know it's fiction, but it's probably still one of the best treatments of the idea out there. Even if such a civilization would have zero interest in our resources we still certainly see no evidence of a star-faring galactic civilization of that type.

Life on larger, slower scales rapidly hits the problem that 15 billion years turns out not to be all that long when you start dividing it by thousands or millions at a time. The problem is that if you're spread out that far, you're probably mostly running at the ambient temperature, which is very cold and much slower, so not only do you need to take 3-6ish orders of magnitude of speed off for dispersion in space, you need to take 3ish orders of magnitude off for being very cold and thus chemistry being very slow. And you may need to take another 3-6+ orders of magnitude off for waiting for your very dispersed, very cold molecules to encounter each other at all, in order to do whatever reactions are supposed to be supporting you. The problem you have there is that on that time scale, the galaxy is a very active and violent place, with supernovae popping off and galactic collisions and passages through the arms and all these stars flying about far faster than you can react to them (rather than the stately procession we think we see) and all kinds of things that tends to make a sensible person thinking about this problem starting thinking that even were some magical force to create such a life form, it wouldn't subjectively live very long before being violently destroyed from its point of view.

(And there are really quite a few more factors missing from the story of how one could have such an enormous life form, like how one forms from what are, to a first approximation, hydrogen atoms with trace contaminants. The biggest "contaminant" also happens to be helium, which you're not going to be doing much chemistry on. Seems difficult to imagine what "chemistry" would be going on here. (And if you want to say "energy being", be concrete about what sort of energy it is you are talking about, and how it manages to accomplish any sort of reaction or computation. It turns out that at least in our universe, "energy being" is probably just a nonsense phrase with no possible physical referent.))

I wouldn't write the idea off perhaps as entirely as some people here have, but if such large organisms or civilizations are going to ever exist, they're going to have to exist in a later epoch of the Universe, i.e., they're still many, many billions of years in the future. Right now the universe is too young and vibrant and explosive for anything like that to exist for very long.

If these sorts of things intrigue you, and you'd like a semi-realistic treatment of some of the similar ideas, you might consider https://www.youtube.com/watch?v=Pld8wTa16Jk . The boundary between "science fiction" and "the optimal possible engineering" is fuzzy but that video does a decent job of riding the line. One of the things I don't think he mentions is that some of the "slow" civilizations he mentions, where a thousand years literally pass like a single thought, is that they might be geographically dispersed, for at least some of their components, on this scale. As you hypothesize beings or civilizations (and honestly, from where we sit, there isn't necessarily that great a difference between the two things) that exist orders of magnitude more slowly than we do, the speed of light stops being such a bummer and "virtually" speeds up.

[1]: https://en.wikipedia.org/wiki/Dragon%27s_Egg