you might have seen a lot of sites with dashes and underscores, for example
1. some-other-site.com has used dashes
2. some_other_sites.com has used underscores.
The question is which one is better,
Google considers dashes or hyphens as separators on a site and underscores doesn’t effect (we can not say if Google changes its policy later)
so I will take above example,
in example 1 it is some other site (as keyword or search result)
and in 2 it is someothersite.
i hope now you understand which is better for you.