You are a wind instrument. You change the pitch by adjusting the length of your pipe. The vocal cord provide the excitation like the reed. You can't change the tightness of your vocal cord for pitch control. But you can totally control the length of pipe to control the pitch.

Your pipe can be as short as just your mouth. It can extend to the throat, mid chest, and as long as your diaphragm.

Try this experiment. Say a high pitch extended "ha". Notice the sound is placed as high as roof of your mouth. Your throat is constricted and closed right behind the tongue.

Now go lower. Say an extended "ha". Feel it originate from your chest. If you are going for a normal deep voice. Practice this. Place your voice at the middle of your chest.

Now if you want Darth Vader low, try to have your voice come from the abdomen. This is not a natural thing people normally do. Try saying "hut, hut, hut" like in football. Place emphasis on your belly button, pushing it out as you say each hut and feel your six packs tighten. Now that you know you can involve your pipe all the way down to the abdomen. It's a matter of practice controlling the windpipe length to get the pitch you want.

Model your speaking voice and pattern after a celebrity like Morgan Freeman or Patrick Stewart.

Eventually your voice will deepen naturally as you age. In the meantime, trying to speak like the captain of a starship will help you to be more aware of your voice.