All verbs which mean “there is” in Greek, go with nominative case. Actually this nominative is their subject.
Υπάρχει Θεός. (There is God).
Είναι κανείς εδώ; (Is there anybody here?)
Γίνεται πανικός. (There is panic.)
Which other verbs go with nominative?
The verbs είμαι (I am), γίνομαι (become), φαίνομαι (I seem to be), ονομάζομαι (I am called/my name is), θεωρούμαι (I am regarded as), εκλέγομαι (I am voted), διορίζομαι (I am appointed) and others with similar meaning, also go with nominative case.
Ο κύριος Κώστας είναι διευθυντής. (Mr Kostas is director)
Ο Γιάννης θεωρείται έξυπνος. (Giannis is considered to be clever)
Ο αδερφός μου εκλέγεται δήμαρχος. (My brother is elected mayor)
This nominative gives a property to the subject and it is called predicate. It should not be confused with the object.