BitcoinDark was originally intended as a means to free traders from the issues present in decentralized exchanges. Traders who intended to make a BitcoinDark investment did so to gain access to their native decentralized asset exchange PAX. This allowed for not only the trading of BTCD, but also for pegged assets without the usual KYC and AML requirements required by many financial institutions.
Sometime in 2016, it was announced that BTCD would be shutting down operations. The developers proposed a coin swap, and 80% of the liquidity of this currency went with them to a new platform, Komodo.
Unfortunately, for those that are still holding their BTCD, they can no longer swap their coins. Strangely, people are still trading this dead asset. Sometimes in the hopes that someone will pick it up again but likely just to manipulate the price.
It's possible to buy BitcoinDark on only a small number of exchanges, this is because the asset is essentially dead. When the developers moved on, they took most of the volume with them. However, some people are still trading it for whatever reason, so if you would like to own it then you can. Bitcoin is the highest volume trading pair for this particular coin, but there is also some US Dollar Tether volume there.
The official wallet is typically the best place to store your coins or tokens for any asset, however, since this coin is no longer being developed you could potentially experience problems that nobody will be there to fix. It's a toss-up whether a no longer supported wallet or the exchange is the best place for these lost coins. Be aware that exchange wallets can go down too, and if there's no developer support then they may never come back online, preventing you from withdrawing or depositing coins.