I have a vector file where I have made some polygons but between the polygons I have empty space.
This entire empty space I would like to make one whole polygon but until now I can't figure out the way to do it.
The easiest way to do this is to create a polygon that covers the extent of your area of interest and turn on "avoid intersections" when drawing it.
I then turn on snapping: go to Settings > Snapping Options... and for Layer selection dropdown choose Advanced.
You must have this set up correctly. I usually recommend a tolerance of 20 pixels, and make sure to tick on "Avoid intersections".
Then you can simply draw a polygon that covers the area you want filled:
Because the new polygon cannot intersect existing ones (you turned on avoid intersections), you get one that fills in just the gaps:
You can do this using the following steps:
1- In QGIS go to
Vector > Research Tools > Polygon From Layer Extent, and use the existing polygon layer as Input File. Choose the Desired name for the output file name of extent layer. This will create a new polygon layer covering all polygons.
Geoprocessing Tools > Difference: Use the extent layer created in step one as input file and the existing polygon layer as difference file. Then You will get an opposite shapefile which covers the empty space.
In the result layer above we have four polygons.