hubieren existido
-they will have existed
Hubieren existido is the future perfect subjunctive form of existir in the third person plural.
See the full existir conjugation

existir

INTRANSITIVE VERB
(to be real)-to exist
Synonyms for existir
INTRANSITIVE VERB
(to be alive)-to live
Synonyms for existir
Want to Learn Spanish?
Spanish learning for everyone. For free.
SpanishDict Premium
Have you tried it yet? Here's what's included:
Cheat sheets
No ads
Learn offline on iOS
Fun phrasebooks
Learn Spanish faster
Support SpanishDict