Details about DARK MATTER
Dark Matter is a Import Boost.
Boosts give you the ability to fly with your Battle-Car and make freestyles and aerial goals. It depends on your Battle-Car how many boost trails your car has. You can equip them inside the customization window in the rocket boost tab.
To get the Dark Matter Boost you can either open a specific crate with a key or trade with other players.
You can search for trade offers with Dark Matter here.