The only way we see ideas manifest is in a physical world, and via minds (predominantly human at that). While ideas are not made out of atoms, they exist within atoms (brains, computers, sound, books and so on). We can imagine ideas exist on their own independent of their manifestation - but its just that - an imagination.
But plenty of ideas can’t be made manifest in the physical world. Like the idea of infinity doesn’t - and can’t - exist in the physical. Yet it’s still a useful idea in mathematics.
And again, the physical cannot be perceived except through a mind of some kind. To play devils advocate, we can imagine the universe existing without a mind to observe it - but it’s just that - an imagination.
