A Spacer is a "virtual" widget, which can be placed into any layout and takes the space a normal widget of the same size would take.

• Spacers are invisible and very light weight because they don't require any DOM modifications

Here are some links that demonstrate the usage of the widget:

• This demo shows how spacers can be used to configure vraiable spacing in a grid.
• This demo shows how spacers can be used to configure vraiable spacing in a box layout.

Here is a link to the API of the Widget:
qx.ui.core.Spacer