The darkroom view is where you develop your images. The center panel contains the image currently being edited.
Middle-click on the center panel to zoom to 1:1 and double-middle-click to zoom to 2:1.
Alternatively you can zoom in and out between 1:1 and “fit to screen” by scrolling with your mouse. Scroll with your mouse while holding the Ctrl key pressed to extend the zoom range to between 2:1 and 1:10.