Learn Russian: Фантастика — это интересно, но мы вряд ли построим машину времени. - Science fiction is interesting, but we will hardly build a time machine.

Вряд ли expresses doubt or extreme unlikelihood, translating to "unlikely" or "hardly." Importantly, it doesn't require any special mood (like the subjunctive бы does). It simply pairs with a standard indicative verb. Here, построим is just the regular perfective future tense meaning "we will build".

I thought фантастика meant "fantasy". Why is it translated as "science fiction"?

Фантастика is a broad term for speculative fiction, but in everyday speech, it's the standard word for "science fiction" (often short for научная фантастика). If you want to specifically talk about the "fantasy" genre (magic, dragons, etc.), Russians use the borrowed word фэнтези.

Why do we use a dash and это, and why is интересно neuter when фантастика is feminine?

The dash plus это construction acts like the "is" in English when defining or commenting on a broad subject. We use the neuter short form интересно because we are making a general, philosophical statement about an abstract concept (science fiction as a whole), rather than describing the direct attributes of a specific feminine noun.

Why is "time" written as времени in машину времени?