Possible Clues:

• Hypothetical stuff in space
• Hypothetical space stuff that doesn't interact with light
• Hypothetical cosmic stuff
• Material forming a large proportion of the mass of the universe
• Sinister business that's apparent throughout the universe
• Astronomers can't directly detect it
• It's made up of WIMPs (Weakly Interacting Massive Particles)
• Debate topic that leads to grim arguments?
• Evil affair — stuff we can't see
• Mysterious and more murky substance?