“I got your big dumb object right here!”
Big Dumb Objects are self-explanatory for the most part. They are objects that are very large and don't do much of anything. Typically they are dangerous (for some reason) and protected from powerful forces of evil. Generally they are more trouble than they're worth having, requiring constant maintenance. However, Big Dumb Objects are expensive and important, with every nation on Earth aspiring to have at least one of them. The bigger the better!
edit Origins of Big Dumb Objects
They happen to be left behind by advanced civilizations a lot. Usually, these "advanced civilizations" once controlled the galaxy and were a violent or peaceful race (toss a coin to decide motivation as the best-of-intentions or worst-of-selfishness). All too often they'd end up wiped out, done in by a mysterious disease.
These diseases were the advanced civilization's fault of course, and they weren't able to stop it either from spreading. The last surviving group makes a frantic abandonment of the contaminated planet but succumbs anyway with their spaceship ending up being a Big Dumb Object that gets in everyones way. Or for something simpler, take an asteroid for example, they just float around space, but are still large enough for someone to waste their time fighting over what to name it. Even though all asteroids are big dumb objects, this is all too commonplace.
The Halo is a prime example of a Big Dumb Object, for the most part, it really does nothing at all. It does spin around the planet, but spinning around other things bigger than itself does not exactly make something noteworthy. Although Jupiter orbits the sun that doesn't stop making it a big ball of gas. The Citadel is just a very confusing building, with a misleading name and physics-defying structures. The only purpose it serves is to be really, really big and for you to shoot things in.
edit Man-Made Objects
Of course we make our own Big Dumb Objects, and we have been making since the time the first scraggly ape-man threw stood on top of a big pile of rocks and tossed sticks and stones at the other ones. The USA has a Big Dumb Object in the State of New York, along it's coast. There have been petitions to declassify it, claiming the Statue of Liberty serves as a lamp for immigrating keebler elves. France has built a very large collection of steel rods into a Big Dumb Object, the French haven't bothered to officially classify it as something yet, simply because they refuse to deal with those bothersome pig-dogs again.
edit Organic Dumb Objects
For things that aren't man made or produced by aliens, Earth manufactures it's own Big Dumb Objects, and subsequently litters them all over the planet. Naturally, since they aren't man-made or extraterrestrial in origin, they are mostly considered to be wastes of space. Blowing up Big Dumb Objects is an expensive and time-consuming task to partake in. Mother Nature cuts demolition workers a break by blowing them up herself once in a while, but this is a rarity and most of the time it must be done the old fashioned way.
These Big Dumb Objects often get into mankind's way. The Titanic happened to run into a Big Dumb Object on its April voyage to Amerika, this made a lot of people very angry and was widely regarded as a bad move. Of course the iceberg got away with it, suffering only minor injuries, the luxury liner was not as lucky though, and sank like a rock. Now the Titanic is a Big Dumb Object resting on the sea floor, and bothering a wide assortment of crustaceans that had otherwise a perfect view of the oceanic plane. The economic value of homes in the area plummeted due to the eyesore and many crabs and crayfish angrily attacked the big dumb object in protest. The ship simply clanged rudely in protest and scared away the plankton in the area that stopped by to get a look at the once-untainted view.
Such long term effects have motivated mankind to plan extreme measures to rid themselves of big dumb objects, the demolition of Mt. Saint Helens protected nearby villages of a possible risk of avalanche, and the Hoover Dam is currently preventing the possibilities of a flash flood from occurring in the area.