The word existiésemos is the imperfect subjunctive form of existir in the first person plural.
See the full existir conjugation
Si no existiésemos ¿Cómo sabría la gente a quién votar?
If we didn't exist, how would people know who to vote for?
Sin materia oscura, el Universo no sería tal como lo conocemos. Quizá ni siquiera existiésemos.
Without dark matter, there would be no Universe as we know it. Perhaps we ourselves would not exist.
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