The Dark Matter Dagger is a powerful dagger that was wielded by Malekith.
Preparing Algrim's Mission
After Malekith discovered that the Aether had found a new host in Asgard, his lieutenant Algrim volunteered to infiltrate the palace and in the process transform into a Kursed. Malekith then used the Dark Matter Dagger to create a wound in Algrim's stomach and inserted a Kurse Stone within.
During the Sacking of Asgard, Malekith tracked Jane Foster to the king's chambers in the Royal Palace of Valaskjalf in order to take the Aether from her. Finding Foster being guarded by Frigga, Asgard's queen, he attempted to duel her in order to get to Foster. Malekith drew the Dark Matter Dagger in order to fight back against Frigga, but Frigga deftly disarmed Malekith, causing the dagger to drop to the ground. The dagger was then left behind in the palace as Malekith and Kurse escaped after successfully managing to kill Frigga.
Alternate Universe Versions
Used by the Collector
The dagger is quite powerful. It can pierce through Asgardian armor with ease. When the Collector used it against T'Challa, the dagger could easily pierce through a Kronan's arm that T'Challa used to protect himself and rip it apart when the Collector dragged T'Challa by the Kronan arm while the dagger was still imbedded in it and pulled it out. The dagger's blade could also extend to the size of an arming sword's blade as shown in Malekith's fight with Frigga.