So, the Breen need refrigeration suits, which also hide them completely from view. They have no blood. They have demonstrated no ethics, morals or loyalties beyond the opportunistic and self-serving. They are empathically or telepathically void. They seem oblivious to physical pain. Their language is mechanical, something no humanoid throat could produce. Their technology is partially biological, demonstrating a knowledge of the bio-sciences far surpassing that of the Federation. They are said to reproduce at a young age–but no one knows what a Breen child looks like. Their homeworld, thought to be a frozen wasteland, is apparently quite temperate...
I think I've just realized what the Breen are.