what was the Xeelee were fighting something even more powerful?

Yes — in the Xeelee Sequence , the Xeelee’s long war was against the Photino Birds , a dark-matter civilization that was even more fundamental in scale than most baryonic life. The conflict is often described as a struggle across the structure of the universe itself, not just a normal space war.

What they were fighting

The Photino Birds are presented as the Xeelee’s main existential rival, and the war is framed as a cosmic clash over the fate of matter, stars, and the universe’s long-term future. In other words, the Xeelee were not just battling another empire — they were opposing something that operated on a deeper physical level.

Why it feels “more powerful”

The Xeelee are already depicted as one of the most advanced baryonic species in Stephen Baxter’s setting, with technology far beyond ordinary civilizations. But the Photino Birds are often described as existing in dark matter and shaping events on a scale that makes the conflict feel almost godlike. That’s why fans usually treat them as the Xeelee’s greatest enemy rather than a conventional rival.

Simple version

• Xeelee = ultra-advanced matter-based species.

• Their biggest enemy = the Photino Birds, a dark-matter intelligence.

• The war = a universe-scale conflict over the cosmic order itself.

So the short answer is: yes, the Xeelee were fighting something even more cosmic in scope, usually identified as the Photino Birds.