You need to add newly create dataGridView1 to your form.
dataGridView1.DataSource = dataTable1;
this.Controls.Add(dataGridView1);
I'm not sure how your text file data arranged, but it seems you are doing it wrong way. for example if you have text file like below
Column1,Column2
data11,data12
data21,data22
then your code should be like below
DataTable dataTable1 = new DataTable();
foreach (string header in headers)
dataTable1.Columns.Add(header, typeof(string), null);
for (int i = 1; i < textData.Length; i++)
dataTable1.Rows.Add(textData[i].Split(','));
1. note that, adding data started from index 1 (second row)
2. when adding row data you can split and set values for each column, otherwise line content will assign to first column only.