Objects in Inkscape can be made partially transparent by manipulating their opacity level, so that the background and the objects below them show through.
The default Canvas background color in Inkscape is fully transparent (zero opacity), and therefore invisible. Although this isn't indicated by a checkerboard pattern like in some other graphics applications, when we export our drawing to the PNG format, the background (space around objects) will be transparent.
Exporting to PNG and transparency
Always use the Export Bitmap option when converting your drawing to the PNG format, and not Save as... | Cairo PNG. The latter may give unexpected results, including an opaque (white) background instead of a transparent ...