Hi Elvis,
Every flyable aircraft has a Centre Of Gravity envelope, mentioned by Snautzer. This is usually stated in the aircraft certification or documentation. The C of G envelope is part of the aircraft limitations and it is specific to the weights and their positions in the aircraft. Very simply, if there is too much weight at the wrong position, it will cause lack of controllability or instability or even excessive stability. Generally, the lighter the aircraft, the more critical the position and weight of anything, such as the Pilot, becomes.
It is possible that some aircraft (very early or total amateur types) exist without documentation of a safe C of G envelope but, it will exist non-the-less or the aircraft will be unflyable in a normal way.
An example was the wood and fabric gliders that I learned to fly at age 15. We had to fly with a removable cast-iron ballast weight in the nose if below a body weight of about 100lbs.
Eng