The Dwarves' fate in Moria was a tragedy in The Lord of the Rings universe, so why didn't Gimli know about it when he arrived with the Fellowship?
Gimli, son of Glóin, doesn't get much time in the spotlight in The Lord of the Rings films. Although his antics with Legolas are fondly remembered as a highlight of the trilogy, Gimli's history is mostly ignored. There's only one real moment in The Fellowship of the Ring that touches upon Gimli's past: when he enters the Mines of Moria in hopes of seeing his cousin, Balin, who was once Lord of Moria.
Unfortunately, this moment is quickly ruined when Gimli realizes his once-flourishing Dwarven realm was overrun and destroyed by Orcs. However, it's strange Gimli wasn't aware of Balin's, and Moria's, grim fate. He only discovered it once the Fellowship of the Ring arrived in the mines, and he saw Balin's tomb. So, why didn't Gimli know about Moria or what happened to his own cousin?
How Did Balin Become Lord of Moria?
Balin was one of the Dwarves who accompanied Thorin Oakenshield in The Hobbit to reclaim the Lonely Mountain, Erebor. Gimli and Balin are cousins and descendants of Durin the Deathless, the first of the noblest house of Dwarves. Balin was well-liked by his peers, who viewed him as wise, kind and always with good counsel. Following the mission to regain Erebor, Balin set off on his own expedition to recover the lost Dwarf Kingdom of Moria, which first fell in the year 1981 of the Third Age. The goal was to re-establish Dwarven rule in the once-majestic underground kingdom and to recover one of the lost Dwarven Rings of Power.
Upon his arrival in Moria, Balin quickly found Durin's Axe and then a Mithril helm. With those in his possession, he declared himself Lord of Moria. However, that may have jinxed the mission. Five years later, in TA 2994, Balin and his entire colony were overrun by Orcs and died within the very halls they tried to resurrect. The only evidence of Balin's time there was the Book of Mazarbul, which contained accounts of the Orc attack and the Dwarf Lord's final moments. That's how Gimli learned what happened to his long-lost cousin.
How Did Gimli Not Know About Moria?
One would think that a disaster like that would be known among the Dwarves, especially considering it was a mission to retake a long-lost Dwarven kingdom. Yet, somehow, Gimli was unaware of Moria's fall until he discovered Balin's tomb. All he knew was that Balin's mission was initially successful, which is why he suggested the Fellowship should travel through the Mines of Moria: He wanted to visit family. Of course, there are potential explanations for why Gimli might have been unaware -- not the least of which is that a messenger dispatched to spread the news was killed by Orcs.
In Peter Jackson's The Lord of the Rings, it's Gimli's ignorance of what happened in Moria that motivates him to visit Balin's kingdom. If Gimli had known the Dwarves were dead, he wouldn't have suggested the shortcut through the mines, and the story would have unfolded much differently. Although he knew the dangers associated with Moria, Gimli was, nevertheless, willing to risk it all just to see his cousins, even at the possible expense of losing Sauron's One Ring.