Its yeast. Unless you want to spend thousands of dollars on a high tech filtration system, you've got to deal with it. Even if you force carb your beer you will still have some. Its a mark of a real homebrew beer. Even some good commercial beers have it.
Its one of the reasons we all pour our bottled beer into glasses before we drink.
After enjoying the beer you can usually just rinse the "gunk" out of the bottle. If you let it dry you can use a bottle brush or an overnight soak in Oxyclean will loosen it so that just a rinse will work.
Before using the bottle make sure it is clean then sanitize well before filling.
yes - but also no. There is a lot of particulate matter in the beer at bottling unless you have filtered, fined, or cold crashed until the beer is clear. Even if you do these things the beer in the bottles is still going to have these solids drop out of solution and settle to the bottom as the beer carbonates and ages.
so the stuff on the bottom is dormant yeast but it can also be left over proteins, hop particulate, and trub from the brewing process.
you can't get rid of it unless you bottle from a carbonated keg - but you can minimize it by cold crashing for a week or more, using irish moss in the boil - whirfloc tablets, filtering out your hops from the boil/using a hop spider, or filtering your beer.
but, there is nothing wrong with the beer - just pour 95% of it out while trying not to disturb the gunk on the bottom and you can have fantastically clear delicious beer.
you can also harvest yeast from the goo on the bottom to use in other batches.