I'm not sure if that works in mid-animation, but here's another way. Right-click in the "void" and choose select all or just select all the objects you have animated with keys. (in some cases you won't have to select EvErYtHiNg) In the time slider, if you hold down shift and drag your curser from one frame to another, you will see that it will highlite in red. You also see an arrow at both far ends of the red bar as well as two arrows in the middle. Using these, you can move keys around, condense (make the selected animation shorter), or expand it (make the animation longer.) All you have to do is move keys to the right and you've got some free space.