It's probably not that the light is losing energy it's just that the distance it travels over time (the time we "know" is supposed to take for a given distance) appears compressed because of unknown/unseen gravitational forces.

Think of it like this: If there were only one star in the universe and it emits a particle of light we could calculate the distance it would travel over time. Yet we know that star will still have a gravitational effect on that light... No matter how far away it gets.

That's what they mean by light "losing energy". Is the energy actually "lost"? Not really. Is this slowing (aka appearance of lost energy) caused by dark energy/dark matter or something more fundamental like spacetime itself being stretched or compressed due to the gravity of astronomical objects we can see or "dark matter"/"dark energy" or... ? We don't really know for certain yet!