Humans have 5 senses, we can see, hear, smell, taste and feel.
There are many things that we cannot see, but it is possible to detect the same using one or more of our other senses.
For example, we cannot see air, but the air can be detected by moving your hand, the air particles strike against the skin and this can be felt. Many gases that are invisible can be detected by their smell. Sound cannot be seen, but we can detect a sound when it strikes against our ear drum. Many salts, when dissolved in water form a solution that looks just like water. They are invisible but can be detected using the sense of taste.
The presence of other things that are invisible to the human eye can be detected using instruments like microscopes or by altering the substances to those that are visible to us.