In design mode, select one of the columns in the chart.
In the Properties pane, look for CustomAttributes; expand that group.
Look for PointWidth within that property group. Change that value from 0.8 (the default) to 0.9 for the columns to be a little closer together. Change the value to 1 if you want your columns to touch. Adjust as desired.
They keep this one hidden, a bit.
Hope this helps.