Remove space (gaps) between multiple lines of flex items when they wrap

An initial setting of a flex container is align-content: stretch.

This means that multiple lines of flex items will be distributed evenly along the cross axis.

To override this behavior, apply align-content: flex-start to the container.

When you're working in a single-line flex container (i.e., flex-wrap: nowrap), the properties to use to distribute space along the cross axis are align-items and align-self.

When you're working in a multi-line flex container (i.e., flex-wrap: wrap) – like in the question – the property to use to distribute flex lines (rows / columns) along the cross axis is align-content.

From the spec:

8.3. Cross-axis Alignment: the align-items and align-self properties

align-items sets the default alignment for all of the flex container’s items, including anonymous flex items. align-self allows this default alignment to be overridden for individual flex items.

8.4. Packing Flex Lines: the align-content property

The align-content property aligns a flex container’s lines within the flex container when there is extra space in the cross-axis, similar to how justify-content aligns individual items within the main-axis. Note, this property has no effect on a single-line flex container.

The align-content property takes six values:

• flex-start
• flex-end
• center
• space-between
• space-around
• stretch

Here's the definition for stretch:

stretch

Lines stretch to take up the remaining space. If the leftover free-space is negative, this value is identical to flex-start. Otherwise, the free-space is split equally between all of the lines, increasing their cross size.

In other words, align-content: stretch on the cross axis is similar to flex: 1 on the main axis.