Even though the Star Wars universe is in near-constant conflict, forcing its planets and races to either fight together or against one another, there are still quite a few mysterious species around. After all, it's difficult, but not impossible, to avoid the civil war tearing the galaxy in half.
These people have either not involved themselves in the greater conversation or they've done a great job at hiding their cards from even their closest allies.
No matter what their motives really are, it doesn't make any curious Star Wars fan less intrigued about their endless piles of secrets.
Kaminoans
During the Clone Wars, the Kaminoans were very involved "allies" of the Republic, giving them legions of clones to fight alongside. However, before their contact with Obi-Wan or Palpatine, Kaminoans were a very cryptic species. Barely anyone knew they existed, or that they were masters of cloning technology.
Their isolationism made them a very mysterious species and, worse, they were wiped out post-Clone Wars. For a short window of time, their people made a great impact on the galaxy and then disappeared all over again.
Jawas
Despite being the well-known junkers of the galaxy, Jawas are otherwise mysterious. They wear their little shrouds, and all people can see are their bright, glowing red eyes. All their lives are hidden in their giant transport ships.
For all the time they spend selling things to Tatooine locals, the closest thing to learning about their culture that Star Wars fans have ever gotten was that time in The Mandalorian that they absolutely lost their minds over communally eating that giant egg. It'd be neat to know what they even talk about.
Cereans
One of the Jedi Grand Masters who led the council during the Clone Wars was Ki-Adi Mundi. A Cerean, he has complex biology with extra brains and hearts. However, when it comes to Cerean culture, history, and even planet-life, the species is practically a mystery. Which is wild, especially considering how influential Ki-Adi Mundi was. He was a wise general, leader, and warrior.
Considering they have a vague history of pollution turning to harmony with nature, it'd be nice to know more about them and their home planet of Cerea.
Chiss
The Chiss hail from the Unknown Regions, meaning they were isolated from the rest of the galaxy. But it probably didn't help their exposure that they also kept to themselves on purpose. Until Admiral Thrawn rose through the Imperial ranks, people knew barely anything about them.
While more was learned about the Chiss from Thrawn, it was an understandably limited perspective from a manipulative villain. However, the Chiss that do leave the Chiss Ascendancy are known for mercenary and spy-work. They almost sound like the Romulans of the Star Wars world.
Dathomirians
More recent Star Wars properties have explored some aspects of life as a Nightsister or Nightbrother of Dathomir, but most of their lives still remain an abject mystery. Fans know they are reclusive, powerful in the force, and and have strict rules for outsiders as well as their own people.
While the galaxy has seen more of the Zabrakians as warriors and Sith (and Ventress' abandonment of her kind) The Nightsisters that lead Dathomir have been aggressively isolationist. These Dathomirians may be some of the strongest force users among the cosmos, but between using Zabrakians to guard their secrets and repelling outsiders, mysteries they remain.
Clawdites
It makes sense that a shape-shifting species would not only be hard to learn about, but be a sort of mystery themselves. However, it's a little impressive how they avoid others. They are introverted by nature and secretive because of persecution on their own planet. After all, Everyone on Zolan are Zolanders, but not all Zolanders are Clawdites.
They may be fearsome bounty hunters, but they need to keep to themselves to survive. It makes sense why they might not have too much of a culture, though what they do have they don't talk much about.
Lasat
Among all these mysterious species, the Lasat are the only ones with any sort of happy ending their mysteries. During the time of the Empire, their people's culture, history, and entire "home" planet Lasan were decimated. Only a few of the warrior species survived the slaughter, which practically erasing their existence.
However, shrouded in the mystery of the Wild Space was the Lasat's true home planet, Lira San, where millions of their people lived. That reveal didn't make them keen to share the secret, though, and the Lasat people still hide their true existence to protect themselves from the dangers of the Empire. (Unless it's to save Lasat refugees, that is).
Hapans
Because of the way the Hapes Cluster exists in the galaxy, they are an isolationist society that is powerful and thriving, but separate from the galaxy. Despite their influence in the Legends stories, though, the universe doesn't know nearly anything about them until everyone has the capability to traverse the dangers of their cluster.
With a half-Nightsister future queen in Legends, as well, the Hapes Cluster is a fascinating place of culture and history that the galaxy deserves more of.
If only their cluster would have revealed their mysteries sooner.
Tusken Raiders
On Tatooine, most all people live in fear of the Tusken Raiders and their brutality. However, other than their dominance over the open sands, locals know little about them and their culture.
Sure, fans may know more from comics and Legends material, but the people of the galaxy? They are just masked boogeymen.
If Tuskens knew how to show the more communal sides of their tribes to the locals, they might be able to be more friendly with them. Instead they are territorial villains with too many knives to expose any mysteries.
Yoda's Species
The captain race of mystery, in the galaxy and for fans, is Yoda's species. For the love of all things Star Wars, fans still don't even know what his race is called. No one knows where his people come from, where they currently are, or if they are all wiped out. Despite Yoda's great influence on the galaxy and the existence of The Child, they still remain hidden from public eye.
Maybe in The Mandalorian someone will finally figure out something about this frustratingly elusive species. They can't all Gandalf off and keep all their secrets.