How do I make XAML DataGridColumns fill the entire DataGrid?

Make sure your DataGrid has Width set to something like {Binding Path=ActualWidth, RelativeSource={RelativeSource Mode=FindAncestor,AncestorType=Window,AncestorLevel=1}}.

Like that, your setting of Width="*" attribute on DataGrid.Columns/DataGridXXXXColumn elements should work.

If you use Width="*" the column will fill to expand the available space.

If you want all columns to divide the grid equally apply this to all columns. If you just want one to fill the remaining space just apply it to that column with the rest being "Auto" or a specific width.

You can also use Width="0.25*" (for example) if you want the column to take up 1/4 of the available width.