BEHOLD! The greatest invention of the 7th millennium!
They probably used a device to disguise themselves from us! Ugh, they think they're SO smart, huh? Now let's start a deadly and costly war to get it!
Hey, where did those really smart people who did our taxes go?
A long time ago in a galaxy far, far away, there lived the great civilization of the Alaky who resided on the planet Philia. The creatures were profoundly advanced for their time and therefore did not want other beings on other planets stealing their knowledge (quite selfish, right?)
Hand it over!
Yeah! what he said!
So being the narcissistic creatures they were, their top scientists developed a new technology that made the planet appear invisible to outsiders.
So this matter has a gravitational force but doesn't interact with light, so we are unable to see it directly. Well, its probably just composed of yet-undiscovered subatomic particles...
The invention worked so well that other planets that already knew of the civilization wanted in on it too. The Alaky, however, were so opposed to sharing the device that the dispute caused a war that seemed to last as long as time itself.
MUHAHAH! Foolish Earthling! That's just what we want you to think!
Eventually, the beings of the other planets emerged victorious and gained the rights to the technology. Ironically, the device that was made to conceal the knowledge of Philia did just the opposite.
Suddenly, thousands of planets disappeared as if they were never in existence before.
And that is precisely the reason why we cannot see "dark matter" (which you know the truth about now). So if your science teacher ever asks you about dark matter, please inform him/her that they are just imperceptible planets who do not want us lowly Earthlings to know of their presence.