I’m not sure aliens / the aliens exist.

Wrong
The (in particular) Grammar Rule
We use the when we are talking about things or people in particular.
> The food in this restaurant is great.

We do NOT use the when we are talking about things or people in general.
> I like cats.
Aha! There is room for improvement. In the next weeks you will keep getting similar question to practice.
Remember the above grammar rule and Try again