So could electrope be a sort of crystallised dark matter that comes out of the rift – like a solidified version of the mist we see around voidgates – and it has formed so much in Alexandria because it has some major "cracks" letting it in?

Though I suppose we should expect to see it in Alzadaal's Legacy as well if that were its source. Though perhaps it would accumulate more quickly in an aetherially thin shard, and with a less contained gate.