I am not a person prone to “see” into the spiritual realm very often, at least not with my physical eyes, but at a very few times in my life, I have seen… with my own eyes,.. Demons. I saw manifestations of what is also called Evil Spirits.
I know this personally by experiential fact but let’s begin here with scriptural reference.
I offer my own witness, but the best witness is the witness of Holy Apostles and Prophets who were given scriptures.
I encourage you to reflect prayerfully and meditate upon the following passages for reliable illumination on this subject.
No commentary below! Just read the passages and ask God for wisdom (James 1:5).
The existence of demons are clearly asserted as Biblical fact:
James 2:19 You believe that there is one God. Good! Even the demons believe that—and shudder
When the demons came out of the man, they went into the pigs, and the herd rushed down the steep bank into the lake and was drowned.
37 Then all the people of the region of the Gerasenes asked Jesus to leave them, because they were overcome with fear. So he got into the boat and left.
38 The man from whom the demons had gone out begged to go with him, but Jesus sent him away, saying,39 “Return home and tell how much God has done for you.” So the man went away and told all over town how much Jesus had done for him.