Dark matter doesn't interact visibily with light or matter. Comets or "random space debris" will not cause consistent 20% dimming.
And yet it's still equally likely, since as the original article, and several posters have pointed out, the whole "it was aliens! Dyson sphere!" thing would be causing large emissions in the IR area, which are not present.
Also, the term "dark matter" is not just applied to exotic invisible space matter, but also to clouds of gas and dust that are just too cold to emit any light (hence, dark). Random space debris in large enough concentrations, oort cloud distortions from another star (the small red dwarf about 130 billion km out that the article mentions) are again a possible cause.
The problems with the majority of these ideas (including the dyson sphere/swarm/...) is that most of them would be showing additional signs like glowing brightly in IR, which is just not present.
Also, the dimming is not a consistent 20% - it's changing frequently, and not in a smooth or repeating pattern. (which would suggest a planet or other orbital body) If anything, what makes it interesting is that it's not consistent, but it keeps happening.