It's the ambiguity of language that's at fault here. The key to the sentence you mentioned is "like normal matter". Normal matter absorbs electromagnetic radiation, increasing its energy level, and drops back to lower energy levels by emitting electromagnetic radiation. Thus, normal matter interacts with light. This is a different physical process than the emission of light due to decay of the particle itself.
And while we haven't pinned down dark matter by any means, it's much more than a stab in the dark. For one, there are known particles--neutrinos--that do not interact via the electromagnetic force, so the idea of unknown particles with the same property isn't unrealistic.
Then, there are clues from many different directions that point to something consistent with matter that interacts gravitationally but not electromagnetically. These include calculations concerning the total matter in the universe, galaxy cluster formation, the rotational speed of stars on the out edge of galaxies, etc.