Only attacked enemy pieces are visible.
Dark1 is played using standard chess rules and start position - the only difference is that you can only see your opponents pieces when they are in your 'searchlight', i.e. squares that your pieces can move to or attack. That is why half of the board looks dark when you start the game. You can see which pieces you have taken by clicking the Material tab.
Your opponents moves are displayed as question marks, however these will still be appended with '+' and '#' for check and mate.
If you return to the game after it is finished you will be able to see the whole board.