A boom could be done a zillion ways. If you have a composer and a full orchestra: Timpani coupled with several double basses playing a quick bowed low note or longer depending on how long you want the boom. A "boom" could be interpreted many ways including collecting real sounds and in post production lower the pitch and cut the sound envelope to make a fast attack. For example, you record a washing machine just before it finishes the spin cycle take the track and cut it right before the final thud, drop the pitch to make it deeper.
The key here is to get a low percussive sound which has the following characteristics: loud, very low pitches, fast attack, and a natural release, even adding a bit of Hall reverb or delay or both will help sweeten up a bit.
Natural boom sounds might be found in thunder, earthquakes, landslides, rock slides, a large tree falling over, etc.
Man made boom sounds can be found in jets breaking the sound barrier, cannons, fireworks, a large truck in a collision, a train wreck, etc.
Alternately, if you have a friend with a synthesizer you might explore some of the pre-programmed samples such as timpani, orchestra hit, Taiko drums, etc.
What I have mentioned are the basic ingredients, stir well, and season to taste. In other words you will likely want to mix a number of sounds to get it just the way you want it.