The existence of the 6 Realms is true and factual in samsara. It is not a mere ideological theory. It has been perceived by many good practitioners in the past. These good practitioners have seen the 6 Realms with their eyes. If these things do not exist, the Buddha will not have talk about them. The Buddha does not need to lie to us.

It is not a good reason to say they do not exist simply because we cannot see them or have not seen them. For example, there is moisture in the air though we cannot see them. Whether we see them or not, there is moisture in the air. Science has yet been able to account for everything in the world.

-- Lati Rinpoche