Once Yol’thanir and The Dark One were the only beings in existence, as the the original pair. Yol’thanir created the Physical realm, out of love and kindness, as The Dark One created the Warp out of wonder and mystery.
But with the creation of their realms, couple with the responsibility to care and nurture them, they felt distant from each other. In a way of solving this issue, both created a link between both realms, a connection per say. Energy from the Warp would sip into the Physical realm and the emotions and lives of the Physical realm would sip into the Warp
Unfortunately, this wasn’t enough, over the eons Yol’thanir and The Dark One became ever more distant, to the point where The Dark One vanished into the warp and Yol’thanir dissipated onto the Physical realm.
But the Bridge still remains.