Well, because folks can't see it, or measure it, or capture it, and thus it is dark, hiding, as it were. Well, matter does not hide. It has no reason to do so, being without a sneaky motive. It is not dark matter that is gluing everything together in some strange, immeasurable way, it is tiny matter, in fact, which the scientists of Earth have yet to see because it is too little for their eyes and their instruments as of yet. The particles of tiny matter, like bugs, are more numerous as they get smaller.
Humans are always horrified to see how many mites there are in a speck of dust.
Mites are everywhere, and should one have the guts to count, the number of mites in the room would be vastly greater than the number of silverfish or cockroaches. And, of course, the number of bugs in the room vastly outweighs the number of humans. Such is the situation in the Universe. Man saw the planets, e.g. humans in the room, and registered their personalities. This happened early on.