I see an empty space in my texture, what is it and why is it there?

The factor that makes changes to this effect is the Unwrap, specifically its utilization. Select a model in RC and a "Selected model(s)" window will appear. Access the Texturing menu in there and you will notice a parameter called Texture utilization (with gutter).

This percentage value represents how well is the UV space filled with UV islands and therefore how much of let's say 8192x8192 pixels are effectively used in the image for the model. The clear gradient in your picture is the empty space of an unwrap. To make your utilization better you can play around with the unwrap styles. Some models are easier for unwrap algorithms to cut them into islands which could perfectly fit in the UV space and some are just harder, it is not always possible to reach the top numbers with some models, so don't be scared if your utilization is not perfect.