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ELI5: In LOTR, how come Gimli had no idea Balin had been dead for so long when they entered Moria?

EDIT: FRONT PAGE HOLY CRAP. 

Thanks!
gpt
In the book, we first meet Gimli and his father Gloin (famous as one of the 13 companions of Thorin Oakenshield) when they've come to Rivendell to seek council. At the Council of Elrond Gloin brings two important pieces of information.

First, he tells how Sauron has sent messengers to Dain (King Under the Mountain following Thorin's death due to severe injuries taken during the Battle of Five Armies) asking for news of a hobbit "that was known to you at one time". The messenger calls the hobbit a thief and claims he stole a ring. They request that Dain give them information to find the thief, and in exchange they will return some of The Seven (I think the offer is 3 of them) to the dwarves. (The Seven refers to the 7 rings of the dwarves referenced in the rhyme at the beginning of the book.)

Gloin tells the council that The Enemy is seeking for news of Bilbo. Information which, combined with the fact that Frodo was pursued by the Nine Riders right to the gates of Rivendell, tells the council that Sauron knows precisely where the One Ring is located. This is obviously bad news.

The other piece of information he brings is about one of the other companions - Balin. He says that several years ago Balin led a group of dwarves to retake Moria. They received news for a while, but things have gone silent. Remember, Middle Earth is a very big place and technologically it's not progressed past swords and horses. The dwarves of the Lonely Mountain have no idea what's happened to Balin and his folk, and they have a very bad feeling about it. However, they also have no proof that anything particularly terrible has happened, and so they have a little hope.

Enter the Fellowship of the Ring. They're headed more or less towards Moria, and Elrond has chosen Gimli to be one of the companions of the Ring. Gandalf's original plan was to ignore Moria entirely and cross over the mountains at the Redhorn Gate. Incidentally, it's mentioned that some of Elrond's scouts used it just a few weeks earlier and found it quite safe. But when they are foiled, they reluctantly decide to enter Moria.

There are several reasons for this. First, they need to get under cover. They've certainly been seen by the birds of Saruman, and need to disappear for a while. Second, the only other fast way to get to Mordor are to walk past Isengard and through Rohan. Obviously Isengard is not an option. Third, they *still hope* that there might be dwarves in Moria, and they'll find Balin alive.

Things are already looking bad after their feet crunch bones walking up the steps into Moria for the first time. And finding nobody alive for the first 2 days of their journey certainly doesn't help. But it's not until they are literally standing next to Balin's tomb and reading the apocalyptic log of their fate that they truly know what's happened.

That's why Gimli is so upset. It's easy to hold out hope when you've only got a bad feeling about things. But to be standing at the tomb, reading "we cannot get out" is quite another thing entirely.