The command is Merge & Center
This command could be easily found in the toolbar of your excel.
If you execute this command in two different cells, those cells will be merged into one larger cell and the content in that cells will be placed exactly in the middle of the combined cell (measures according to combined cells' length)
Negative space is the space between, within and surrounding an object in an image. The positive space is the focus of the image, the object itself, but the negative space is just as important. It shares edges with the positive space, defining the outline of the object and creating proportion