Dark elixir is a resource in clash of clans, you can use it from town hall level 7. When you are town hall level 7 you can build a dark elixir storage but no dark elixir pump. Meaning you can only raid dark elixir. From town hall level 8 you can build a dark elixir pump, allowing you to collect it yourself.
Dark elixir that is collected is stored in the dark elixir storage.
With dark elixir you can:
Dark elixir is a huge part of the game strategy from town hall level 8. Lower level player usually don’t know what it is and what it can do.