In Thor: Ragnarok, Hela refers Odin stopped at 9th realm while they were conquering all that there is, and she says that Odin banished her after that.
Does that mean that there is more to the universe than nine realms?
In comics, Marvel do have more than 9 realms.
9 realms were
- Alfheim Home of the Light Elves. Alfheim is a distinct region on the Asgard planetoid.
- Asgard Home of the Asgardians. Asgard is the name of the planetoid, a distinct region on the planetoid and its capital city.
- Hel Realm of the dead who are neither honored nor dishonored. Ruled by Hela.
- Jotunheim Home of the Frost Giants.
- Midgard The Earthly plane. Although not technically a part of the Asgardian dimension, it is considered one of the Nine Worlds because of its significant connections to Asgard.
- Muspelheim Home of the Demons. Ruled by Surtur.
- Nidavellir Home of the Dwarves. Nidavellir is a distinct region on the Asgard planetoid.
- Svartalfheim Home of the Dark Elves.
- Vanaheim Home of the Vanir who are the sister race of the Asgardians. Vanaheim is a distinct region on the Asgard planetoid.
But there were additional realms too:
- Niffleheim Realm of the dishonored dead which is distinct but closely connected to Hel.
- Valhalla Realm of the honored dead and is a distinct region on the Asgard planetoid.
- Nornheim realm of the Asgard planetiod ruled by Karnilla
- Heven The tenth world of Asgard and home of the Angels, it was cut off from the rest of Asgard by Odin long ago and not rediscovered until the 2014 storyline Original Sin.
But none of those additional realms appeared in MCU films/TV shows yet. Or Hela might be referring to other alien planets or other dimension which are not part of Norse mythology or realms.
But in MCU Niffleheim and Hel is same and part of same realm.
Also remember, The Nine Realms are a group of distant planets that are interconnected by the cosmic nimbus Yggdrasil and are home to various different races and cultures. So there can be other realms which have no connection to Yggdrasil. And Hela wanted to rule everything. src