That also goes for 'aliens', because presumably, they'd also be made of normal matter, and if they're not, then we're back to some exotic form of matter like dark matter.
As far as being a mistake, it could be possible that we are fundamentally misunderstanding something about how gravity works. That would be as interesting a result as dark matter, to be honest.
I assume by this you mean dust and planets and such. https://medium.com/starts-with-a-bang/could-dark-matter-just...
That also goes for 'aliens', because presumably, they'd also be made of normal matter, and if they're not, then we're back to some exotic form of matter like dark matter.
As far as being a mistake, it could be possible that we are fundamentally misunderstanding something about how gravity works. That would be as interesting a result as dark matter, to be honest.