Darwin Gabriel Núñez Ribeiro was born in Artigas, Artigas Department, Uruguay, to an impoverished family. His father, Bibiano Núñez, was a builder, and his mother, Silvia Ribeiro, was a hawker of milk bottles.

Núñez was scouted by former Uruguay international José Perdomo and moved to Montevideo to join Peñarol's youth academy.

Núñez suffered a cruciate ligament injury that required two operations and kept him out for over a year. During this time, his older brother Junior quit football to provide for the family.