From The Encyclopedia Eorzea, Volume I:
Though one could walk a thousand malms and never reach the other, the void and the Source lie but a hair’s breadth apart, separated by an unseen veil serving to prevent travel between the two worlds. This veil, however, is not absolute, and with the right amount of aetherial force can be torn to create temporary gateways - gateways to which aether-starved voidsent are drawn, sensing the life energies of our realm. Naturally occurring rents are typically minor, allowing only the smallest and weakest of voidsent through before the wound heals. However in the years leading up to and following the Calamity, the barrier between the two planes has grown weak, resulting in larger and more frequently tears and thusly stronger intruders.
Roleplay Note: As most voidsent player characters keep that fact secret, this category is hidden so it does not show up on character pages.