Expanders usually push the back teeth outwards (towards the cheeks). If the patient's jaw is growing, a gap may appear between the front teeth that wasn't there before. This is because the growth zone of the upper jaw is in the middle of the palate – where its left and right halves meet – and a gap appearing between the two middle teeth shows that new bone has formed. You may see this effect with a so-called rapid expander – one that is activated every day for a set period. With slow expanders, the teeth have time to shift and fill the space that opens slowly and gradually between them, before you even get the chance to notice it.
This answer is for general information. Your own case needs an examination by an orthodontist.