Why are glucose and galactose considered enantiomers?

Diastereomers are molecules that have 2 or more stereogenic centers and differ at some of these centers with respect to absolute configurations. This disqualifies them from being mirror images of each other.

If we examine the figure below, we can see in the center, the straight chain configuration of glucose has the aldehyde #"carbon"# numbered #1#, because aldehydes are given a higher priority when naming, according to IUPAC rules.

To the right of #"D-Glucose"#, its diastereomer, #"D-Galactose"#, looks deceptively identical to it. However, the highlighted pink portion will tell you that at #"Carbon"# number 4, the absolute configuration is different. #color(orange)("In D-Glucose, the OH group is facing the right side")# while on the #color(red)("D-Galactose, the OH at carbon 4 is facing the left side")#. Every other #"carbon"#, besides #"carbon"# number #4#, is exactly the same in respect to their absolute configurations.

Both are still polyhydroxy aldolases because they both contain an aldehyde as a functional group (aldolase) and both contain many hydroxyl groups (polyhydroxy).