Finding the portal, like many great scientific discoveries, was complete accident.
When she first found it she thought she had accidentally created a mirror from nothing. She could see herself staring back at her, but then she started having a conversation with herself and they realised that they had both stumbled upon a window to a parallel universe.
News of it spread far and wide. If you decided to go and have a look you would always find yourself staring back, because obviously the Other You would have made the same decision.
Scientists ran tests to work out the difference between the two worlds and every night Our World would go to bed buzzing with excitement.
Every night the Other World would unzip their reflective skin and calculate how long it would take to break through the barrier.