Other commenters are telling you nonsense about how Descartes argues for an external world.

His argument in the Meditations first establishes, as you and others said, that he exists but then goes from there to establish that he has an idea of an infinite being, an idea that he argues could only come from an actual infinite being that exists independently of his own mind (basically, the idea's content is too much to have ever come from any finite being like himself).

From there, he establishes that this being must have created him and must be good, so would not have created him with mental faculties that would be unable to detect their own errors. Since a systematic falsity of perception would be an undetectable error, our senses must not be systematically false. Therefore, at least some of the external objects we perceive must exist and any mistakes we make about what objects are actually out there must be able to be corrected, as we do in natural philosophy.

Yeah, the argument seems to be that a mind cannot exist without a universe to contain it but that assumes we know the nature of the mind/universe, unless I'm missing something.