I've always wondered this. Clearly the two know about each other, what with two invasions from Akavir and the Avakiri Blades who protect the Emperor? (Not actually Akaviri, heavily influenced)

So I've always wondered, why does nobody know anything about Akavir in Tamriel? Well, not that much, anyway. There is the one book, 'Mysterious Akavir' which shows how little they know in an in-you-face obvious way.

Why not send some Imperial missionaries there to have a look around? Maybe see if it a viable place for conquest? Or trade? Or to assess whether or not anything there is a threat to Tamriel or the Empire?

Wouldn't it make sense to at least try and understand more about this place?

A combination of factors, mostly.

The biggest one to remember is up until 3E 433, there were expeditions to Akivir - so theoretically trade between the two continents was also possible. After the Oblivion Crisis (Where there's no information on whether or not Akivir was even affected), Tamriel was too busy trying to keep itself from plunging into outright chaos to be bothered with diplomatic missions across the sea.

Another major factor to realize is Akivir is a continent filled with Beast races. No men live there. That makes it a very "alien" place to the rulers of Tamriel - to say nothing of the inherent racism of men and mer regarding Beast Races if the treatment of Argonians and Khajiit is anything to go by.

It's very far away, so it's difficult and expensive to get there. Also, the last time Tamriel tried to conquer Akavir was a disaster (read "Disaster at Ionith") that lost them an Emperor. I think the invasion of Akavir was a shock to the Third Empire because not since Tiber Septim's time had they met a power that could contend with them, and their superiority did not extend to Akavir, where the rules were different.

I thought the Akavir were Men? (This is the ONE article on a race I've not looked up)

Uriel the... fifth or fourth?... tried to conquer parts of it. Even established a couple of colonies. They didn't last very long.

I take two factors that relate to each other from real life logistics and the notion from the civilization games.

While the that are on Tamriel and Akavir can help reach other, they're not the best suited things for deep/far ocean travel. Trading in Tamriel uses "shallow" water boats and that's about the extent of their technology. Remember, even though the Vikings made it over to North America, they did so using routes that are relatively shallow closer to the arctic circle and easier to navigate than crossing the wide open Atlantic.

Simply put, the sea-faring ships they have aren't ideal for a consistent trade, only colonization/invasion.

There were men once...then the race of Vampiric Snake People ate them all.

You've also got a indigenous monkey race, a race of Ice Demons that thaw out every Spring that the monkeys beat back, and a race of Tiger people ruled over by a god-king who managed to transform himself from a Tiger person into a Dragon and who make war with the Vampiric Snake Men.